Add support for running the specs with JRuby

Hi all!

I have now gotten the specs to run with JRuby and ARJDBC:

* PostgreSQL:  205 examples, 29 failures
* SQLite3:  136 examples, 36 failures
* MySQL:  Still som setup error.  I don't use it.

All the JRuby spec runs are marked as "Allowed failures" in .travis.yml, so the specs run GREEN.

I will continue to work on this, but it would be a great motivation for me if you could merge this PR as a base line for further work.