Get specs running under Rubinius

Add rubinius-specific dependencies to get specs running.  Taken from blog post here - http://www.benjaminfleischer.com/2013/12/05/testing-rubinius-on-travis-ci/

Specs still fail on array equality, presumably for similar reasons as discussed here - https://github.com/jruby/jruby/issues/1291 - for JRuby, but they at least run.