Setup rbenv for managing Ruby environments
Merge branch 'master' into mac
Configure Ruby bundler to install gems locally

Links a config file to `~/.bundle/config` that tells bundler to install
gems locally in `vendor/` and to not re-use gems.

This is based on http://dan.carley.co/blog/2012/02/07/rbenv-and-bundler/
Merge branch 'master' into mac