Rubinius and JRuby changes to Gemfile and .travis.yml.

1. Updated the Gemfile to include Rubinius gems
2. Added JRuby and Rubinius to the .travis.yml
3. Changed enum_values scope definition to use a Proc, so it passes against Rails master.
4. Added a begin/rescue block around RDocTask, as that task isn't currently supported on Rails 4+ in Rubinius