rvm plugin: Fix `gems` to work with GNU sed
rvm plugin: Fix `gems` to work with GNU sed

Fix #1579.

FrankSansC/oh-my-zsh,nunogt/oh-my-zsh,tekniklr/oh-my-zsh,FrankSansC/oh-my-zsh,tekniklr/oh-my-zsh,FrankSansC/oh-my-zsh,FrankSansC/oh-my-zsh,lightmanhk/oh-my-zsh,lightmanhk/oh-my-zsh,lightmanhk/oh-my-zsh,nunogt/oh-my-zsh,lightmanhk/oh-my-zsh