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`.
this was also copied right from https://github.com/travis-ci/docs-travis-ci-com/pull/133/files#diff-935618eb408c0327454a931718613410R13 just with the names changed