Get specs running under Rubinius

1. Added Rubinius gems to Gemfiles.  
2. Updated Rakefile to handle Rubinius/Rails 4 case where RDocTask is not available.

Almost all of the test matrix is green with this PR.  There is still one failure - ActiveRecord edge + Rubinius.  This appears to be the result of a recently introduced incompatibility.  I'm not sure how to fix it.  But the remaining Rubinius specs all run as expected now.