Use nvm as it works great

Not using n since nvm by default does better by installing to home
directory and maintaining global packages within that. Easier to delete
and reinstall that way just like rbenv.
Add rbenv-update and rbenv-gem-rehash plugins

Rework script so it only installs if rbenv and plugins don’t exist
since the rbenv-update plugin takes care of updating rbenv and any
installed plugins (awesome!!). Also with rbenv-gem-rehash no need to
ever have to run rehash anymore as it does automatically whenever
installing a gem.