Handle no rbenv scenario in ruby main.pp template.
Handle no rbenv scenario in ruby main.pp template.

I did this so that we can handle the scenario where the users don't want
shanty to manage their ruby versions and instead just want to use the system
version of ruby that is already setup in their VM via custom puppet manifests.

justinleveck/boxci,codebreakdown/boxci,reachlocal/boxci