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.
Potentially there shall be more community-supported build scripts, in which case we'll want to extract this part to, say `Travis::Build::Script#announc_community_support` or some such. (Well, you can leave this PR as is; I'll take a look at this extraction later.)