Switch from RVM to Rbenv
Upgrade to Chef 11
Upgrade rbenv cookbook
Install ruby as the vagrant user
Always add /opt/rbenv/shims to the PATH

As an annoying side-effect, when running `vagrant provision` for the
second time the Chef gem will be installed into the global Rbenv ruby
instead of Vagrant's ruby.
Update README for Rbenv
Merge pull request #68 from gitlabhq/rbenv

Switch from RVM to Rbenv