Use gpg by default rather than gpgv for signature verification

Current gpgv has a bug where the gpgv command does not work

A patch has already been sent upstream :
https://lists.gnupg.org/pipermail/gnupg-devel/2015-December/030623.html

Using gpg --verify rather than gpgv fixes the issue.
If gpg is not found, gpgv is used as a fallback.