Your rvm is probably not up to date, and by doing so the list of installable rubies also. Rvm probably took the best match, that was Rubinius 2
When you do
rvm list known
You obtain the list of known rubies that you can install
This list is updated at the same time you update rvm. Then you need to update rvm before installing MRI Ruby 2.0.0
rvm get stable