skipping gpg verification of rvm in vagrant (gpg1 instead of gpg2 package installed)

[emanuelb]

Steps to reproduce

Install metasploit using vagrant by the script at #8693

Current behavior

==> default: Downloading https://github.com/rvm/rvm/archive/1.29.2.tar.gz
==> default: Downloading https://github.com/rvm/rvm/releases/download/1.29.2/1.29.2.tar.gz.asc
==> default: Found PGP signature at: 'https://github.com/rvm/rvm/releases/download/1.29.2/1.29.2.tar.gz.asc',
==> default: but no GPG software exists to validate it, skipping.

There is open issue about it in:
rvm/rvm#4059
which suggest to use gnupg2 package (GPG2 instead of GPG1) to fix it.