Starting from v2.1 passing passwords directly to gpg does not work
anymore [1], instead a loopback interface must be used otherwise
gpg >2.1 will error out with:
"gpg: signing failed: Inappropriate ioctl for device"
gpg <2.1 does not work with the new --pinentry-mode arg and gives an
invalid option error, so we detect what is the running version of gpg
and pass it accordingly.
