Enable JRuby support for AR

Right now this is just an experiment, I'm enabling JRuby support for
ActiveRecord (with `allow_failures` option on Travis), we'll see how it goes.
temporarily set jruby tests only for rails4
Add jruby builds for mongoid and mongo_mapper (Rails 4.x)
Merge pull request #588 from arnvald/experimental_jruby_support

Enable JRuby support for AR