Fix rbenv gems helper
Fix rbenv gems helper

Fixes gem versions with words (beta, pre) in the version or with
multiple installed versions not being highlighted, and compatibility
with GNU sed (related to robbyrussell/oh-my-zsh#1579).
ammula88/oh-my-zsh,ammula88/oh-my-zsh,ammula88/oh-my-zsh,ammula88/oh-my-zsh