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.
Nice catch, verified it with the command below and patching now:

    mkdir -p ~/julia && curl -s -L https://julialang.s3.amazonaws.com/bin/linux/x64/0.3/julia-0.3.3-linux-x86_64.tar.gz  | tar -C ~/julia --strip-components=1 -x -z -f -