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.
also in RVM the check order is reversed to `which gpg2 || which gpg` - although they are supposed to be compatible the v2 might be safer one day