Use new generic install script in default Travis

Ref #7364 and https://github.com/JuliaLang/julialang.github.com/issues/151 - I put together a shell script to handle installing binaries on different platforms, now I'm proposing to use it for the default `.travis.yml` from `Pkg.generate`. I've got a [otherwise unfinished] test package using this script across a matrix of linux and mac on both release and nightlies [here](https://travis-ci.org/tkelman/BLIS.jl/builds/38539365), though it looks like the osx builds are going to be queued for a while.

Also add the `os:` section for osx testing (has to be explicitly enabled by sending an email to support@travis-ci.com, see http://docs.travis-ci.com/user/multi-os, it doesn't do anything if it hasn't been enabled yet).