RFC: Create generic julia install script

Closes https://github.com/JuliaLang/julia/issues/7364 - mostly intended for use on Travis and similar CI services, so you can do something like `curl http://julialang.org/install-julia.sh | sh -s $JULIAVERSION` and have it work for both Linux and Mac if you've enabled multi-OS support.

Generic Linux binary download links are apparently subject to change. I think the Windows side of this can be done properly if we set up Chocolatey (https://github.com/JuliaLang/julia/pull/7386).