Fix travis config file and add html-proofer test

.travis.yml

@@ -1,4 +1,22 @@
-#!/usr/bin/env bash
-set -e # halt script on error
+language: ruby
+rvm:
+  - 2.1
+
+before_script:
+  - chmod +x ./scripts/cibuild # or do this locally and commit
+
+# Assume bundler is being used, therefore
+# the `install` step will run `bundle install` by default.
+script: ./scripts/cibuild
+
+# branch whitelist, only for GitHub Pages
+branches:
+  only:
+    - gh-pages # test the gh-pages branch
+
+env:
+  global:
+    - NOKOGIRI_USE_SYSTEM_LIBRARIES=true # speeds up installation of html-proofer
+
+sudo: false # route your build to the container-based infrastructure for a faster build
 
-bundle exec jekyll build

Gemfile

@@ -1,3 +1,4 @@
 source "https://rubygems.org"
 
 gem "jekyll"
+gem "html-proofer"

scripts/cibuild

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+set -e # halt script on error
+
+bundle exec jekyll build
+bundle exec htmlproofer ./_site --only-4xx --allow-hash-href