Switch the default version of GNU Privacy Guard back to 2.0.

This patch effectively undoes @wkennington's update from 850da18. The problem
with GnuPG 2.1 is that its built-in 1.x compatibility differs from the one
provided in version 2.0, and these changes break 'gpg1compat in ways we don't
understand yet.

Commit 4c84621 attempted to remedy that issue by fixing gpg1compat to GPG 2.0,
but this caused further trouble because the GPG agent from versions 2.0 and 2.1
are incompatible, so users of the GPG 1.x interface (who were using GPG 2.0)
could not talk to their X session's agent (which comes from GPG 2.1).

The corresponding ticket is https://github.com/NixOS/nixpkgs/issues/4888.