Use GPG for rvm verification.

rvm will fail to install if GPG is present on the system, but the key for rvm is not found.
This behaviour will be skipped if the `gpg_key` node is an empty string.
Fixes #278

Based on this fix, https://github.com/IndependentIP/chef-rvm/commit/56f2459edfdd43385a5b30c703f6f84f861b87e2, by @marijnm.
Ah right, duh, thanks.