Add Action Cable tests to travis matrix

.travis.yml

@@ -16,6 +16,7 @@ env:
   matrix:
     - "GEM=railties"
     - "GEM=ap"
+    - "GEM=ac"
     - "GEM=am,amo,as,av,aj"
     - "GEM=ar:mysql"
     - "GEM=ar:mysql2"

ci/travis.rb

@@ -23,6 +23,7 @@ class Build
     'ar'       => 'activerecord',
     'av'       => 'actionview',
     'aj'       => 'activejob',
+    'ac'       => 'actioncable',
     'guides'   => 'guides'
   }