Switch from RVM to Rbenv

An alternative to https://github.com/gitlabhq/gitlab-vagrant-vm/pull/67 .

Update: I ended up upgrading Chef to version 11.6 in order to be able to use the latest version of the rbenv cookbook.

### Things that I would like to see working
- [x] `vagrant destroy && vagrant up`
- [x] green build using Ruby 1.9.3
- [x] green build using Ruby 2.0.0
- [x] check that gitlab-shell uses the right Ruby `ssh vagrant@vagrant_ip '/usr/bin/env ruby -v'`