Remove jruby from travis until failure is resolved

The build is error on jruby while trying to run bundle install. I haven't been able to figure out why, so I'm disabling it for now.

If anyone really cares about jruby, you're welcome to dig into it. https://travis-ci.org/bkeepers/dotenv/jobs/37002101

/cc @petergoldstein https://github.com/bkeepers/dotenv/pull/83