add rvm wrapper

A wrapper is needed for rvm as described in
https://raw.github.com/lunks/fish-nuggets/master/functions/rvm.fish

Because I already had an ~/config/fish/functions directory I installed
it with the following command in the ~/config/fish/functions/ directory

curl -o rvm.fish https://raw.github.com/lunks/fish-nuggets/master/functions/rvm.fish