Add Travis CI

@mrsimo @bjhess Seems like the easiest way to guarantee that https://github.com/harvesthq/time-warp/pull/11 doesn't slip in the future is to automatically check for it.

This adds Travis with a matrix that covers a pretty wide spread of Rubies (same ones I use for bcrypt-ruby).

jruby-head kept failing with a thing that I didn't really feel like debugging, so right now the matrix is allowing failures for that.  ¯\\\_(ツ)_/¯