Remove JRuby from .travis.yml

Sort of works, but it doesn’t seem to be using rationals correctly, e.g.:

1) Rounding #floor_to should be 2014-09-05 22:11:05 +0000 for Time 2014-09-05 22:11:05 +0000 step by (1/1000)
     Failure/Error: expect(result).to eq(expected)
       
       expected: 2014-09-05 22:11:05.284000000 +0000
            got: 2014-09-05 22:11:05.284000071 +0000