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
Make sure gpg installs keys to /root/.gnupg explicitly

Was using /home/deploy/.gnupg instead, probably due to the use of sudo to run chef-solo.