cloudfoundry_source does not install rbenv

[andreacampi]

From Fabrizio Frosali

Deploying with run list:

recipe[cloudfoundry-dea]

errors out:

rbenv not installed. Can't run rbenv_command


# In /var/chef/cache/cookbooks/cloudfoundry/providers/source.rb

 54:     gr = rbenv_gem "bundler" do
 55:       version new_resource.bundler_version
 56:       ruby_version new_resource.ruby_version
 57:       action :nothing
 58:     end
 59:     gr.run_action(:install)

[andreacampi]

It would be nice to fix this so that cloudfoundry_source can install its own dependencies, but that's not an easy fix.

All other cookbooks install rbenv for themselves, so I'm going to do the same here.
We can improve that later and clean all the cookbooks up.

[andreacampi]

On second thought: we still want to be able to specify the version of ruby that we want, as we do in every other cookbook.
So the code that we have may not be ideal (and could be factored out into a library) but it's not really incorrect.