gpgme 1.5.1

Big version bump for gpgme, and nudges it over to requiring gpg2 rather
than gpg. The build documentation for gpgme have been strongly
recommending this for quite some time, and MacPorts, Debian and Ubuntu
made this change a while back.
Merge branch 'gpgme' of https://github.com/DomT4/homebrew into gpgme