fix issues with default values

Regexing undef will not match undef, only 'undef' - this resulted in
actual_version = undef, which meant the RVM installation failed.
only attempt to upgrade if RVM is already installed

Previously, if you set a version or latest, then both the initial install and
the upgrade would attempt to run, with the latter failing due to the notify
resource being missing. Moved the upgrade into the check that $::rvm_version
was set