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.
Good point. So far there's been pretty little desire for anything aside from latest release or latest nightly, but that might change. The URL's for fixed releases are a bit messier, and we haven't been doing generic binaries for more than one or two releases. Still not a bad idea though.