Add feature to install rbenv and ruby-build from tarball

Hi

I've added the possibility to install rbenv and ruby-build from a tarball instead of git, since I'm deploying stuff on machines without internet access. Would you be interested in this? In this version I actually include the tarballs in the cookbook, which might be a bad idea, I'm going to look into changing it so it can be set up from a "wrapper" cookbook instead. 

If you're interested in this functionality, I'll clean it up a bit and update this PR, otherwise I'll just hack it to work for me. So I wouldn't really recommend merging this in it's current shape.