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
Check for presence of gpg2 as well as gpg.
Apparently some servers may have the former instead of the latter.
Checking for gpg2 requires that we also *actually use* that command.
Defaulting to gpg2, for parity with rvm.
Merge pull request #285 from Headlinerfm/verify-with-gpg

Use GPG for rvm verification.