JRuby compatibility

Hi, I was thinking of using Paper Trail in a project that runs in JRuby. I noticed that JRuby isn't in the .travis.yml, so I modified some things so the project can bundle and run the tests in JRuby. It looks like the library itself works perfectly. I just had to make a modification to one test (the custom serializer) and some of the gems (JRuby has its own sqlite3 adapter you have to use).

Let me know what you think. Everything is green in Travis.