Newer versions of debian and ubuntu come with gpg pointing to gpg2.
We can currently only handle gpg1 CLIs though. Luckily the old gpg is still
available in the package gnupg1 (providing bin/gpg1).
As a bit of a stop-gap, until #657 can be resolved properly, we'll detect
the version of bin/gpg. If it is unsuitable we'll fall back and try
bin/gpg1. If neither is found to be suitable the signer/verifier will
not work.
Same applies to gpgv/gpgv1.
