Add rbenv_prefix_for helper method.
Add rbenv_prefix_for helper method.

transportapi/rbenv-cookbook,cloudfoundry/rbenv-cookbook,newrelic-forks/chef_rbenv,transportapi/rbenv-cookbook,transportapi/rbenv-cookbook,docwhat/rbenv-cookbook,RiotGamesCookbooks/rbenv-cookbook,newrelic-forks/chef_rbenv,docwhat/rbenv-cookbook,newrelic-forks/chef_rbenv,docwhat/rbenv-cookbook,RiotGamesCookbooks/rbenv-cookbook,cloudfoundry/rbenv-cookbook