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.
Thanks, I just spotted that I missed your last comment.  Rust does the following:

    it "downloads and installs Rust" do
      is_expected.to run(/dist\/rust-nightly.*\.tar\.gz/)
    end

If I understand RSpec correctly, this checks for the given expression in some sort of command log?  If so, then we would need something like:

    it 'downloads and installs Julia'
      is_expected.to run /\/stable\/linux-x86_64/
    end

Am I correct in this?