JRuby compatibility and Travis integration for existing Mongoid 3.0 branch

I didn't notice the existing Mongoid 3.0.x branch when I submitted my first pull request. So I'm pulling the relevant code over to this branch - namely ensuring that JRuby runs green and adding Travis support.

The Travis matrix is much simpler, as Mongoid 3.0.x only works with Ruby 1.9.3 or JRuby in 1.9 mode. The base repo will still need to be Travis CI enabled, as described here, to get automated CI tests running.

Please let me know if you have any questions. Thanks.

(Resubmitting against the correct base branch)