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.
Recall that `JL_PKG` is evaluated on the build instance, but this script is compiled on a separate machine ahead of time. My previous comment (https://github.com/travis-ci/travis-build/pull/318#issuecomment-65824683) was erroneous.

I think that
`sh.if '-f src/${JL_PKG}.jl' do`
should work here, and the subsequent two occurrences of `JL_PKG`.