Editing rbenv in mac to fix zsh error

Clean install on OSX Mavericks 10.9.4, I encountered an error similar to this one: https://github.com/sstephenson/rbenv/issues/487

Implemented same zsh fix, seemed to work alright. I think this is what needs to be changed in the code. The closed rbenv issue seems to say it was fixed on their end back in November, so maybe just a path needs updating, too.