Document Julia support

ref https://github.com/travis-ci/travis-build/pull/318

cc @ninjin @staticfloat @StefanKarpinski @ViralBShah please comment if any feedback

There's a slight alphabetization inconsistency with the placement of JavaScript vs Node.js in `user/getting-started.md`.
Funnily enough, "in the" is correct here.  English treats online discussion areas like rooms, hence the phrase "chat room", or "forum" (which itself refers to a [large common space](http://en.wikipedia.org/wiki/Forum_(Roman))).