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`.
We could use:

    https://github.com/travis-ci/travis-ci/issues/new?labels=julia

For most people it will be a nop, but at least the Travis-CI people will attach the correct label without too much of an issue.