Julia support

Adding initial [Julia](http://julialang.org/) support.  Julia and Travis has been a love story for quite some time, with Travis powering the majority of testing for both the official language implementation as well as a good chunk of the available packages.  This PR attempts to bring Julia support to Travis.  As I this is my first attempt at writing Ruby code, so I suspect that things may be off here and there and any and all feedback is very much appreciated.  The tests passed locally using Vagrant, but `spec` is certainly the part which I have the least understanding of, please, do kick the tires.
I've just created a new label `julia`, so that you can filter the issues better. You can use `https://github.com/travis-ci/travis-ci/issues/new?labels=julia` to pre-apply this label to the issue users will open.