rvm plugin: update to ruby version helpers and rvm-update

* the current patch levels hard-coded here are pretty dated. I updated the
ruby version helpers to use loose ruby version matchers so they don't
continually need to be updated with every new patch level release.

* `rvm get head` actually performs an `rvm reload` in the post install.
there's no need to do again here in rvm-update so I've removed that.