jRuby & Rubinius support

Support for jRuby and Rubinius was added by Brian Hempel in his [fork](https://github.com/brianhempel/mongo_session_store). It wasn't merged in PR #13 after updates in [my fork](https://github.com/appsignal/mongo_session_store). Will try to add support later.

- [ ] Add support for jRuby
- [ ] Add support for Rubinius
- [ ] Fix any issues that might occur
- [ ] Add tests to TravisCI matrix