install ruby2.0 by using rvm

According to the NEWS, ruby 2.0.0-p0 is released in February. With the performance improvement, lower memory usage, feature enhancement, and back support to 1.9.3, Everyone is encouraged to upgrade to ruby2.0.

And here is the method about how to upgrade ruby under rvm:

rvm get head
rvm install 2.0.0
rvm use 2.0.0

Happy hacking, and keep fun~