Test Travis matrix config

.travis.yml

@@ -1,4 +1,6 @@
-language: ruby
+matrix:
+  include:
+  - language: ruby
     rvm:
       - 2.4.1
 
@@ -9,3 +11,12 @@ script: ./build.sh
     sudo: false # route your build to the container-based infrastructure for a faster build
 
     cache: bundler # caching bundler gem packages will speed up build
+
+  - language: node_js
+    node_js:
+      - "v12.7.0"
+    branches:
+      only:
+        - master
+    script:
+      - "npm run html-beautify"