Add JRuby to the supported ruby version

The travis-ci tests run against both jruby-18 and jruby-19, they are not allowed failures and both are green.
Seems to me, like not adding them was just an oversight. If it isn't I'd like to know the reasoning :-)

Cheers,
Tobi
Merge pull request #612 from PragTob/patch-1

Add JRuby to the supported ruby version