Added travis CI

Signed-off-by: Jim Martens <github@2martens.de>

Gemfile.lock

@@ -19,6 +19,7 @@ GEM
       execjs
     coffee-script-source (1.11.1)
     colorator (1.1.0)
+    colorize (0.8.1)
     commonmarker (0.17.7.1)
       ruby-enum (~> 0.5)
     concurrent-ruby (1.0.5)
@@ -36,6 +37,15 @@ GEM
     html-pipeline (2.7.1)
       activesupport (>= 2)
       nokogiri (>= 1.4)
+    html-proofer (3.7.6)
+      activesupport (>= 4.2, < 6.0)
+      addressable (~> 2.3)
+      colorize (~> 0.8)
+      mercenary (~> 0.3.2)
+      nokogiri (~> 1.8.1)
+      parallel (~> 1.3)
+      typhoeus (~> 0.7)
+      yell (~> 2.0)
     i18n (0.9.1)
       concurrent-ruby (~> 1.0)
     jekyll (3.6.2)
@@ -162,6 +172,7 @@ GEM
       mini_portile2 (~> 2.3.0)
     octokit (4.7.0)
       sawyer (~> 0.8.0, >= 0.5.3)
+    parallel (1.12.1)
     pathutil (0.16.1)
       forwardable-extended (~> 2.6)
     popper_js (1.12.9)
@@ -194,6 +205,7 @@ GEM
     tzinfo (1.2.4)
       thread_safe (~> 0.1)
     unicode-display_width (1.3.0)
+    yell (2.0.7)
 
 PLATFORMS
   ruby
@@ -202,6 +214,7 @@ DEPENDENCIES
   activesupport
   autoprefixer-rails
   bootstrap (~> 4.0.0.beta2)
+  html-proofer
   jekyll
   jekyll-assets
   jekyll-avatar