Every time you need to upgrade to a new Ruby, you need to upgrade RVM first. This is because your current RVM won't know which Rubies are available. To do this, upgrade your RVM to head by using the following command. Note that this will just upgrade the RVM environment in your home directory, and won't touch the RVM gem itself.
Note that to update to the head branch, RVM needs to check the latest version of itself out of a Git repository. To do this, it needs to be able to access the Git command line program. Make sure that's installed first. It's installed by installing the git-core package on Ubuntu Linux.
$ sudo apt-get install git-core
$ rvm update --head