RVM 1.18.15 and Ruby 1.9.3-p392

* RVM 1.18.15 adds the [recently released](http://www.ruby-lang.org/en/news/2013/02/24/ruby-2-0-0-p0-is-released/) Ruby 2.0.0-p0
* Ruby 1.9.3-p392 contains [security fixes](http://www.ruby-lang.org/en/news/2013/02/22/ruby-1-9-3-p392-is-released/) so should probably be our new default.