script for updating ruby versions by executing rbenv update-versions

Added a way to update all versions by just typing `rbenv update-versions` from anywhere in the terminal without setting an rbenv path.

_After trying to add this script to rbenv where it was misplaced because it uses the plugin, I try to add this little script in the Plugin itself._

PR try for the rbenv respo: https://github.com/sstephenson/rbenv/pull/406