JRuby compatibility

I'm thinking of using Tire on a project that will run in JRuby. I noticed the .travis.yml didn't include JRuby, so I  thought I'd try to get the tests running under JRuby. I was successful - the unit and integration tests all run successfully under JRuby.

This patch modifies the gem spec and test framework so it will bundle and run under JRuby. No change to the actual Tire implementation was necessary.