rvm upgrade

Install new ruby, copy gemsets, make gems pristine, remove old rubies

Subcommands

Name	Description
`rbx`	Rubinius
`jruby`	JRuby
`ruby`	MRI/YARV Ruby (The Standard), defaults to 1.8.6
`ree`	Ruby Enterprise Edition
`system`	Use the system ruby (eg. pre-RVM state)
`default`	Use RVM set default ruby and system if it hasn't been set