Switches to publicly-available "rouge" gem

Gemfile was previously pointed to a local path for "rouge".  Instead use the public gem.  (Needed this to be able to test locally.)

If you're using local "rouge" modifications, it'd be nice to get them pushed upstream.  If you're just using an as-yet-unreleased gem, maybe [:github and/or :ref](http://bundler.io/v1.3/git.html) would be a better alternative.