robbyrussell/oh-my-zsh — comment on commit a5545bd31c76a4f34339dc4e7b87f74fa0174015
File:  line 0
By peterhoeg:

rbenvdir is used to detect the root of the rbenv installation. We only use it to mangle the PATH if rbenv is not on it.

If rbenv is in /usr/local/bin then it's already in the path and we have no problems.