chore: add Node.js 12 to CI services (#430)

diff --git a/.travis.yml b/.travis.yml
index 4105e07..6e1447c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,9 +8,10 @@
     - $HOME/.npm
 
 node_js:
-  - "6"
-  - "8"
-  - "10"
+  - 6
+  - 8
+  - 10
+  - 12
 
 before_script:
   # Ensure that bin script actually runs on used Node.js version
diff --git a/appveyor.yml b/appveyor.yml
index f601739..bf13e43 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -7,9 +7,10 @@
 
 environment:
   matrix:
-    - nodejs_version: "6"
-    - nodejs_version: "8"
-    - nodejs_version: "10"
+    - nodejs_version: 6
+    - nodejs_version: 8
+    - nodejs_version: 10
+    - nodejs_version: 12
 
 install:
   - ps: Install-Product node $env:nodejs_version