Update GCC48 version to 4.8.3 and add boxen prefix

I saw @wfarr [removed the boxen prefix when he vendored the homebrew recipe](https://github.com/boxen/puppet-gcc/commit/85e1dade615b79f2de6d4997b9a8a0d06be6438b) but I believe it is better to follow the pattern we are using to vendored recipes in all the modules.

BTW, I think we can safely remove the `sighs` branch now that I merged it on master.

cc @dgoodlad @tarebyte