Update gems and remove gem system update

* Require ruby 2.6 or later
* Ruby 2.6 comes with bundler as a default gem, so remove any system
  updating or installation of bundler in Travis CI, because the shipped
  version should be fine as long as Ruby 2.6 or later is used
diff --git a/.travis.yml b/.travis.yml
index d1b1282..cb0c989 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,8 +1,6 @@
 language: ruby
 rvm:
   - 2.6
-before_install:
-  - gem update --system
 branches:
   except:
     - asf-site
diff --git a/Gemfile b/Gemfile
index 779380b..ef20a63 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,3 +1,4 @@
+ruby '>2.6'
 source 'https://rubygems.org'
 gem 'jekyll', '>= 3.7.4'
 gem 'jekyll-redirect-from', '>= 0.13.0'
diff --git a/Gemfile.lock b/Gemfile.lock
index aa451f5..547941e 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -9,7 +9,7 @@
       eventmachine (>= 0.12.9)
       http_parser.rb (~> 0.6.0)
     eventmachine (1.2.7)
-    ffi (1.11.1)
+    ffi (1.11.3)
     forwardable-extended (2.6.0)
     http_parser.rb (0.6.0)
     i18n (1.7.0)
@@ -40,7 +40,7 @@
       kramdown (~> 2.0)
     libv8 (3.16.14.19)
     liquid (4.0.3)
-    listen (3.2.0)
+    listen (3.2.1)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.3.6)
@@ -51,7 +51,7 @@
     rb-inotify (0.10.0)
       ffi (~> 1.0)
     ref (2.0.0)
-    rouge (3.11.1)
+    rouge (3.14.0)
     safe_yaml (1.0.5)
     sassc (2.2.1)
       ffi (~> 1.9)
@@ -70,5 +70,8 @@
   jekyll-redirect-from (>= 0.13.0)
   therubyracer
 
+RUBY VERSION
+   ruby 2.6.5p114
+
 BUNDLED WITH
    1.17.2