Choose ruby version with included rvm function

I was using my own version for two reasons:

1. I didn't like the way rvm handled .rvmrc files.
2. My custom pbrewrc check broke the built-in rvm cd alias.

But now:

1. RVM now supports the much nicer .ruby-version/.ruby-gemset format
2. Using the internal function that rvm uses to read these files means I
can use them alongside my pbrewrc customizations.