Upgrade nokogiri to version 1.8.2 (#246)

*Motivation*

Version <1.8.2 contain a security vulnerability.
diff --git a/website/Gemfile b/website/Gemfile
index da77425..2583055 100644
--- a/website/Gemfile
+++ b/website/Gemfile
@@ -1,5 +1,6 @@
 source 'http://rubygems.org'
 gem 'jekyll'
+gem 'nokogiri', '1.8.2'
 gem 'jekyll-redirect-from'
 gem 'html-proofer'
 gem 'RbST'
diff --git a/website/Gemfile.lock b/website/Gemfile.lock
index 1ab11b5..6011223 100644
--- a/website/Gemfile.lock
+++ b/website/Gemfile.lock
@@ -2,73 +2,87 @@
   remote: http://rubygems.org/
   specs:
     RbST (0.5.1)
-    activesupport (4.2.7)
-      i18n (~> 0.7)
-      json (~> 1.7, >= 1.7.7)
+    activesupport (5.2.0)
+      concurrent-ruby (~> 1.0, >= 1.0.2)
+      i18n (>= 0.7, < 2)
       minitest (~> 5.1)
-      thread_safe (~> 0.3, >= 0.3.4)
       tzinfo (~> 1.1)
-    addressable (2.4.0)
+    addressable (2.5.2)
+      public_suffix (>= 2.0.2, < 4.0)
     colorator (1.1.0)
-    colored (1.2)
-    ethon (0.9.0)
+    colorize (0.8.1)
+    concurrent-ruby (1.0.5)
+    em-websocket (0.5.1)
+      eventmachine (>= 0.12.9)
+      http_parser.rb (~> 0.6.0)
+    ethon (0.11.0)
       ffi (>= 1.3.0)
-    ffi (1.9.14)
+    eventmachine (1.2.7)
+    ffi (1.9.25)
     forwardable-extended (2.6.0)
-    html-proofer (3.0.6)
-      activesupport (~> 4.2)
+    html-proofer (3.9.1)
+      activesupport (>= 4.2, < 6.0)
       addressable (~> 2.3)
-      colored (~> 1.2)
+      colorize (~> 0.8)
       mercenary (~> 0.3.2)
-      nokogiri (~> 1.5)
+      nokogiri (~> 1.8.1)
       parallel (~> 1.3)
-      typhoeus (~> 0.7)
+      typhoeus (~> 1.3)
       yell (~> 2.0)
-    i18n (0.7.0)
-    jekyll (3.2.1)
+    http_parser.rb (0.6.0)
+    i18n (0.9.5)
+      concurrent-ruby (~> 1.0)
+    jekyll (3.8.3)
+      addressable (~> 2.4)
       colorator (~> 1.0)
+      em-websocket (~> 0.5)
+      i18n (~> 0.7)
       jekyll-sass-converter (~> 1.0)
-      jekyll-watch (~> 1.1)
-      kramdown (~> 1.3)
-      liquid (~> 3.0)
+      jekyll-watch (~> 2.0)
+      kramdown (~> 1.14)
+      liquid (~> 4.0)
       mercenary (~> 0.3.3)
       pathutil (~> 0.9)
-      rouge (~> 1.7)
+      rouge (>= 1.7, < 4)
       safe_yaml (~> 1.0)
-    jekyll-redirect-from (0.11.0)
-      jekyll (>= 2.0)
-    jekyll-sass-converter (1.4.0)
+    jekyll-redirect-from (0.14.0)
+      jekyll (~> 3.3)
+    jekyll-sass-converter (1.5.2)
       sass (~> 3.4)
-    jekyll-watch (1.5.0)
-      listen (~> 3.0, < 3.1)
-    json (1.8.3)
-    kramdown (1.11.1)
-    liquid (3.0.6)
-    listen (3.0.8)
+    jekyll-watch (2.0.0)
+      listen (~> 3.0)
+    kramdown (1.17.0)
+    liquid (4.0.0)
+    listen (3.1.5)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
+      ruby_dep (~> 1.2)
     mercenary (0.3.6)
-    mini_portile2 (2.1.0)
-    minitest (5.9.0)
-    nokogiri (1.6.8)
-      mini_portile2 (~> 2.1.0)
-      pkg-config (~> 1.1.7)
-    parallel (1.9.0)
-    pathutil (0.14.0)
+    mini_portile2 (2.3.0)
+    minitest (5.11.3)
+    nokogiri (1.8.2)
+      mini_portile2 (~> 2.3.0)
+    parallel (1.12.1)
+    pathutil (0.16.1)
       forwardable-extended (~> 2.6)
-    pkg-config (1.1.7)
-    rb-fsevent (0.9.7)
-    rb-inotify (0.9.7)
-      ffi (>= 0.5.0)
-    rouge (1.11.1)
+    public_suffix (3.0.2)
+    rb-fsevent (0.10.3)
+    rb-inotify (0.9.10)
+      ffi (>= 0.5.0, < 2)
+    rouge (3.1.1)
+    ruby_dep (1.5.0)
     safe_yaml (1.0.4)
-    sass (3.4.22)
-    thread_safe (0.3.5)
-    typhoeus (0.8.0)
-      ethon (>= 0.8.0)
-    tzinfo (1.2.2)
+    sass (3.5.6)
+      sass-listen (~> 4.0.0)
+    sass-listen (4.0.0)
+      rb-fsevent (~> 0.9, >= 0.9.4)
+      rb-inotify (~> 0.9, >= 0.9.7)
+    thread_safe (0.3.6)
+    typhoeus (1.3.0)
+      ethon (>= 0.9.0)
+    tzinfo (1.2.5)
       thread_safe (~> 0.1)
-    yell (2.0.6)
+    yell (2.0.7)
 
 PLATFORMS
   ruby
@@ -78,6 +92,7 @@
   html-proofer
   jekyll
   jekyll-redirect-from
+  nokogiri (= 1.8.2)
 
 BUNDLED WITH
-   1.12.5
+   1.16.2
diff --git a/website/docs/0.4.0-incubating/Gemfile b/website/docs/0.4.0-incubating/Gemfile
index 8bc5038..3979fb2 100644
--- a/website/docs/0.4.0-incubating/Gemfile
+++ b/website/docs/0.4.0-incubating/Gemfile
@@ -1,5 +1,6 @@
 source 'https://rubygems.org'
 gem 'jekyll'
+gem 'nokogiri', '1.8.2'
 gem 'jekyll-redirect-from'
 gem 'html-proofer'
 gem 'RbST'
diff --git a/website/docs/0.4.0-incubating/Gemfile.lock b/website/docs/0.4.0-incubating/Gemfile.lock
index 136ae6f..6011223 100644
--- a/website/docs/0.4.0-incubating/Gemfile.lock
+++ b/website/docs/0.4.0-incubating/Gemfile.lock
@@ -1,74 +1,88 @@
 GEM
-  remote: https://rubygems.org/
+  remote: http://rubygems.org/
   specs:
     RbST (0.5.1)
-    activesupport (4.2.7)
-      i18n (~> 0.7)
-      json (~> 1.7, >= 1.7.7)
+    activesupport (5.2.0)
+      concurrent-ruby (~> 1.0, >= 1.0.2)
+      i18n (>= 0.7, < 2)
       minitest (~> 5.1)
-      thread_safe (~> 0.3, >= 0.3.4)
       tzinfo (~> 1.1)
-    addressable (2.4.0)
+    addressable (2.5.2)
+      public_suffix (>= 2.0.2, < 4.0)
     colorator (1.1.0)
-    colored (1.2)
-    ethon (0.9.0)
+    colorize (0.8.1)
+    concurrent-ruby (1.0.5)
+    em-websocket (0.5.1)
+      eventmachine (>= 0.12.9)
+      http_parser.rb (~> 0.6.0)
+    ethon (0.11.0)
       ffi (>= 1.3.0)
-    ffi (1.9.14)
+    eventmachine (1.2.7)
+    ffi (1.9.25)
     forwardable-extended (2.6.0)
-    html-proofer (3.0.6)
-      activesupport (~> 4.2)
+    html-proofer (3.9.1)
+      activesupport (>= 4.2, < 6.0)
       addressable (~> 2.3)
-      colored (~> 1.2)
+      colorize (~> 0.8)
       mercenary (~> 0.3.2)
-      nokogiri (~> 1.5)
+      nokogiri (~> 1.8.1)
       parallel (~> 1.3)
-      typhoeus (~> 0.7)
+      typhoeus (~> 1.3)
       yell (~> 2.0)
-    i18n (0.7.0)
-    jekyll (3.2.1)
+    http_parser.rb (0.6.0)
+    i18n (0.9.5)
+      concurrent-ruby (~> 1.0)
+    jekyll (3.8.3)
+      addressable (~> 2.4)
       colorator (~> 1.0)
+      em-websocket (~> 0.5)
+      i18n (~> 0.7)
       jekyll-sass-converter (~> 1.0)
-      jekyll-watch (~> 1.1)
-      kramdown (~> 1.3)
-      liquid (~> 3.0)
+      jekyll-watch (~> 2.0)
+      kramdown (~> 1.14)
+      liquid (~> 4.0)
       mercenary (~> 0.3.3)
       pathutil (~> 0.9)
-      rouge (~> 1.7)
+      rouge (>= 1.7, < 4)
       safe_yaml (~> 1.0)
-    jekyll-redirect-from (0.11.0)
-      jekyll (>= 2.0)
-    jekyll-sass-converter (1.4.0)
+    jekyll-redirect-from (0.14.0)
+      jekyll (~> 3.3)
+    jekyll-sass-converter (1.5.2)
       sass (~> 3.4)
-    jekyll-watch (1.5.0)
-      listen (~> 3.0, < 3.1)
-    json (1.8.3)
-    kramdown (1.11.1)
-    liquid (3.0.6)
-    listen (3.0.8)
+    jekyll-watch (2.0.0)
+      listen (~> 3.0)
+    kramdown (1.17.0)
+    liquid (4.0.0)
+    listen (3.1.5)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
+      ruby_dep (~> 1.2)
     mercenary (0.3.6)
-    mini_portile2 (2.1.0)
-    minitest (5.9.0)
-    nokogiri (1.6.8)
-      mini_portile2 (~> 2.1.0)
-      pkg-config (~> 1.1.7)
-    parallel (1.9.0)
-    pathutil (0.14.0)
+    mini_portile2 (2.3.0)
+    minitest (5.11.3)
+    nokogiri (1.8.2)
+      mini_portile2 (~> 2.3.0)
+    parallel (1.12.1)
+    pathutil (0.16.1)
       forwardable-extended (~> 2.6)
-    pkg-config (1.1.7)
-    rb-fsevent (0.9.7)
-    rb-inotify (0.9.7)
-      ffi (>= 0.5.0)
-    rouge (1.11.1)
+    public_suffix (3.0.2)
+    rb-fsevent (0.10.3)
+    rb-inotify (0.9.10)
+      ffi (>= 0.5.0, < 2)
+    rouge (3.1.1)
+    ruby_dep (1.5.0)
     safe_yaml (1.0.4)
-    sass (3.4.22)
-    thread_safe (0.3.5)
-    typhoeus (0.8.0)
-      ethon (>= 0.8.0)
-    tzinfo (1.2.2)
+    sass (3.5.6)
+      sass-listen (~> 4.0.0)
+    sass-listen (4.0.0)
+      rb-fsevent (~> 0.9, >= 0.9.4)
+      rb-inotify (~> 0.9, >= 0.9.7)
+    thread_safe (0.3.6)
+    typhoeus (1.3.0)
+      ethon (>= 0.9.0)
+    tzinfo (1.2.5)
       thread_safe (~> 0.1)
-    yell (2.0.6)
+    yell (2.0.7)
 
 PLATFORMS
   ruby
@@ -78,6 +92,7 @@
   html-proofer
   jekyll
   jekyll-redirect-from
+  nokogiri (= 1.8.2)
 
 BUNDLED WITH
-   1.12.5
+   1.16.2
diff --git a/website/docs/0.5.0/Gemfile b/website/docs/0.5.0/Gemfile
index 8bc5038..3979fb2 100644
--- a/website/docs/0.5.0/Gemfile
+++ b/website/docs/0.5.0/Gemfile
@@ -1,5 +1,6 @@
 source 'https://rubygems.org'
 gem 'jekyll'
+gem 'nokogiri', '1.8.2'
 gem 'jekyll-redirect-from'
 gem 'html-proofer'
 gem 'RbST'
diff --git a/website/docs/0.5.0/Gemfile.lock b/website/docs/0.5.0/Gemfile.lock
index 136ae6f..6011223 100644
--- a/website/docs/0.5.0/Gemfile.lock
+++ b/website/docs/0.5.0/Gemfile.lock
@@ -1,74 +1,88 @@
 GEM
-  remote: https://rubygems.org/
+  remote: http://rubygems.org/
   specs:
     RbST (0.5.1)
-    activesupport (4.2.7)
-      i18n (~> 0.7)
-      json (~> 1.7, >= 1.7.7)
+    activesupport (5.2.0)
+      concurrent-ruby (~> 1.0, >= 1.0.2)
+      i18n (>= 0.7, < 2)
       minitest (~> 5.1)
-      thread_safe (~> 0.3, >= 0.3.4)
       tzinfo (~> 1.1)
-    addressable (2.4.0)
+    addressable (2.5.2)
+      public_suffix (>= 2.0.2, < 4.0)
     colorator (1.1.0)
-    colored (1.2)
-    ethon (0.9.0)
+    colorize (0.8.1)
+    concurrent-ruby (1.0.5)
+    em-websocket (0.5.1)
+      eventmachine (>= 0.12.9)
+      http_parser.rb (~> 0.6.0)
+    ethon (0.11.0)
       ffi (>= 1.3.0)
-    ffi (1.9.14)
+    eventmachine (1.2.7)
+    ffi (1.9.25)
     forwardable-extended (2.6.0)
-    html-proofer (3.0.6)
-      activesupport (~> 4.2)
+    html-proofer (3.9.1)
+      activesupport (>= 4.2, < 6.0)
       addressable (~> 2.3)
-      colored (~> 1.2)
+      colorize (~> 0.8)
       mercenary (~> 0.3.2)
-      nokogiri (~> 1.5)
+      nokogiri (~> 1.8.1)
       parallel (~> 1.3)
-      typhoeus (~> 0.7)
+      typhoeus (~> 1.3)
       yell (~> 2.0)
-    i18n (0.7.0)
-    jekyll (3.2.1)
+    http_parser.rb (0.6.0)
+    i18n (0.9.5)
+      concurrent-ruby (~> 1.0)
+    jekyll (3.8.3)
+      addressable (~> 2.4)
       colorator (~> 1.0)
+      em-websocket (~> 0.5)
+      i18n (~> 0.7)
       jekyll-sass-converter (~> 1.0)
-      jekyll-watch (~> 1.1)
-      kramdown (~> 1.3)
-      liquid (~> 3.0)
+      jekyll-watch (~> 2.0)
+      kramdown (~> 1.14)
+      liquid (~> 4.0)
       mercenary (~> 0.3.3)
       pathutil (~> 0.9)
-      rouge (~> 1.7)
+      rouge (>= 1.7, < 4)
       safe_yaml (~> 1.0)
-    jekyll-redirect-from (0.11.0)
-      jekyll (>= 2.0)
-    jekyll-sass-converter (1.4.0)
+    jekyll-redirect-from (0.14.0)
+      jekyll (~> 3.3)
+    jekyll-sass-converter (1.5.2)
       sass (~> 3.4)
-    jekyll-watch (1.5.0)
-      listen (~> 3.0, < 3.1)
-    json (1.8.3)
-    kramdown (1.11.1)
-    liquid (3.0.6)
-    listen (3.0.8)
+    jekyll-watch (2.0.0)
+      listen (~> 3.0)
+    kramdown (1.17.0)
+    liquid (4.0.0)
+    listen (3.1.5)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
+      ruby_dep (~> 1.2)
     mercenary (0.3.6)
-    mini_portile2 (2.1.0)
-    minitest (5.9.0)
-    nokogiri (1.6.8)
-      mini_portile2 (~> 2.1.0)
-      pkg-config (~> 1.1.7)
-    parallel (1.9.0)
-    pathutil (0.14.0)
+    mini_portile2 (2.3.0)
+    minitest (5.11.3)
+    nokogiri (1.8.2)
+      mini_portile2 (~> 2.3.0)
+    parallel (1.12.1)
+    pathutil (0.16.1)
       forwardable-extended (~> 2.6)
-    pkg-config (1.1.7)
-    rb-fsevent (0.9.7)
-    rb-inotify (0.9.7)
-      ffi (>= 0.5.0)
-    rouge (1.11.1)
+    public_suffix (3.0.2)
+    rb-fsevent (0.10.3)
+    rb-inotify (0.9.10)
+      ffi (>= 0.5.0, < 2)
+    rouge (3.1.1)
+    ruby_dep (1.5.0)
     safe_yaml (1.0.4)
-    sass (3.4.22)
-    thread_safe (0.3.5)
-    typhoeus (0.8.0)
-      ethon (>= 0.8.0)
-    tzinfo (1.2.2)
+    sass (3.5.6)
+      sass-listen (~> 4.0.0)
+    sass-listen (4.0.0)
+      rb-fsevent (~> 0.9, >= 0.9.4)
+      rb-inotify (~> 0.9, >= 0.9.7)
+    thread_safe (0.3.6)
+    typhoeus (1.3.0)
+      ethon (>= 0.9.0)
+    tzinfo (1.2.5)
       thread_safe (~> 0.1)
-    yell (2.0.6)
+    yell (2.0.7)
 
 PLATFORMS
   ruby
@@ -78,6 +92,7 @@
   html-proofer
   jekyll
   jekyll-redirect-from
+  nokogiri (= 1.8.2)
 
 BUNDLED WITH
-   1.12.5
+   1.16.2
diff --git a/website/docs/latest/Gemfile b/website/docs/latest/Gemfile
index 8bc5038..3979fb2 100644
--- a/website/docs/latest/Gemfile
+++ b/website/docs/latest/Gemfile
@@ -1,5 +1,6 @@
 source 'https://rubygems.org'
 gem 'jekyll'
+gem 'nokogiri', '1.8.2'
 gem 'jekyll-redirect-from'
 gem 'html-proofer'
 gem 'RbST'
diff --git a/website/docs/latest/Gemfile.lock b/website/docs/latest/Gemfile.lock
index 136ae6f..6011223 100644
--- a/website/docs/latest/Gemfile.lock
+++ b/website/docs/latest/Gemfile.lock
@@ -1,74 +1,88 @@
 GEM
-  remote: https://rubygems.org/
+  remote: http://rubygems.org/
   specs:
     RbST (0.5.1)
-    activesupport (4.2.7)
-      i18n (~> 0.7)
-      json (~> 1.7, >= 1.7.7)
+    activesupport (5.2.0)
+      concurrent-ruby (~> 1.0, >= 1.0.2)
+      i18n (>= 0.7, < 2)
       minitest (~> 5.1)
-      thread_safe (~> 0.3, >= 0.3.4)
       tzinfo (~> 1.1)
-    addressable (2.4.0)
+    addressable (2.5.2)
+      public_suffix (>= 2.0.2, < 4.0)
     colorator (1.1.0)
-    colored (1.2)
-    ethon (0.9.0)
+    colorize (0.8.1)
+    concurrent-ruby (1.0.5)
+    em-websocket (0.5.1)
+      eventmachine (>= 0.12.9)
+      http_parser.rb (~> 0.6.0)
+    ethon (0.11.0)
       ffi (>= 1.3.0)
-    ffi (1.9.14)
+    eventmachine (1.2.7)
+    ffi (1.9.25)
     forwardable-extended (2.6.0)
-    html-proofer (3.0.6)
-      activesupport (~> 4.2)
+    html-proofer (3.9.1)
+      activesupport (>= 4.2, < 6.0)
       addressable (~> 2.3)
-      colored (~> 1.2)
+      colorize (~> 0.8)
       mercenary (~> 0.3.2)
-      nokogiri (~> 1.5)
+      nokogiri (~> 1.8.1)
       parallel (~> 1.3)
-      typhoeus (~> 0.7)
+      typhoeus (~> 1.3)
       yell (~> 2.0)
-    i18n (0.7.0)
-    jekyll (3.2.1)
+    http_parser.rb (0.6.0)
+    i18n (0.9.5)
+      concurrent-ruby (~> 1.0)
+    jekyll (3.8.3)
+      addressable (~> 2.4)
       colorator (~> 1.0)
+      em-websocket (~> 0.5)
+      i18n (~> 0.7)
       jekyll-sass-converter (~> 1.0)
-      jekyll-watch (~> 1.1)
-      kramdown (~> 1.3)
-      liquid (~> 3.0)
+      jekyll-watch (~> 2.0)
+      kramdown (~> 1.14)
+      liquid (~> 4.0)
       mercenary (~> 0.3.3)
       pathutil (~> 0.9)
-      rouge (~> 1.7)
+      rouge (>= 1.7, < 4)
       safe_yaml (~> 1.0)
-    jekyll-redirect-from (0.11.0)
-      jekyll (>= 2.0)
-    jekyll-sass-converter (1.4.0)
+    jekyll-redirect-from (0.14.0)
+      jekyll (~> 3.3)
+    jekyll-sass-converter (1.5.2)
       sass (~> 3.4)
-    jekyll-watch (1.5.0)
-      listen (~> 3.0, < 3.1)
-    json (1.8.3)
-    kramdown (1.11.1)
-    liquid (3.0.6)
-    listen (3.0.8)
+    jekyll-watch (2.0.0)
+      listen (~> 3.0)
+    kramdown (1.17.0)
+    liquid (4.0.0)
+    listen (3.1.5)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
+      ruby_dep (~> 1.2)
     mercenary (0.3.6)
-    mini_portile2 (2.1.0)
-    minitest (5.9.0)
-    nokogiri (1.6.8)
-      mini_portile2 (~> 2.1.0)
-      pkg-config (~> 1.1.7)
-    parallel (1.9.0)
-    pathutil (0.14.0)
+    mini_portile2 (2.3.0)
+    minitest (5.11.3)
+    nokogiri (1.8.2)
+      mini_portile2 (~> 2.3.0)
+    parallel (1.12.1)
+    pathutil (0.16.1)
       forwardable-extended (~> 2.6)
-    pkg-config (1.1.7)
-    rb-fsevent (0.9.7)
-    rb-inotify (0.9.7)
-      ffi (>= 0.5.0)
-    rouge (1.11.1)
+    public_suffix (3.0.2)
+    rb-fsevent (0.10.3)
+    rb-inotify (0.9.10)
+      ffi (>= 0.5.0, < 2)
+    rouge (3.1.1)
+    ruby_dep (1.5.0)
     safe_yaml (1.0.4)
-    sass (3.4.22)
-    thread_safe (0.3.5)
-    typhoeus (0.8.0)
-      ethon (>= 0.8.0)
-    tzinfo (1.2.2)
+    sass (3.5.6)
+      sass-listen (~> 4.0.0)
+    sass-listen (4.0.0)
+      rb-fsevent (~> 0.9, >= 0.9.4)
+      rb-inotify (~> 0.9, >= 0.9.7)
+    thread_safe (0.3.6)
+    typhoeus (1.3.0)
+      ethon (>= 0.9.0)
+    tzinfo (1.2.5)
       thread_safe (~> 0.1)
-    yell (2.0.6)
+    yell (2.0.7)
 
 PLATFORMS
   ruby
@@ -78,6 +92,7 @@
   html-proofer
   jekyll
   jekyll-redirect-from
+  nokogiri (= 1.8.2)
 
 BUNDLED WITH
-   1.12.5
+   1.16.2