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`.
Could this be more of a "This language is community-supported" kind of warning? Or should there be something like that to go along with it? While it's cool for us to have it be in beta, it should also be explicitly market as not officially supported for the future.