Add rabl-rails support (Depends on earlier fix_specs pull request)

Add rabl-rails support and tests for the new feature. gon/rabl.rb now gracefully attempts to require 'rabl' or 'rabl-rails', suppressing any LoadError raised. Specs for gon.rabl testing now use helper methods defined in spec_helper.rb to ensure either rabl or rabl-rails is loaded at any given time.