Add ability to install rbenv independently

- Can now install rbenv separately. Can be used as a requirement in a
  state for installing rbenv plugins using the git or file states
Fix description of method and whitespace lost
Merge branch 'develop' into add-rbenv-install-as-state
Merge pull request #11110 from amco/add-rbenv-install-as-state

Add rbenv.install_rbenv as separate state