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`.
Whoops I guess we might not count as "first class support" in which case I could remove the changes to this file. C# had additions here but D didn't.