bugfix: gpg2 is called 'gpg' on macOS

The gpg2 command isn't always around; it's sometimes called gpg.  This is
the case with the brew-installed version, and it's breaking our tests.

- [x] Look for both 'gpg2' and 'gpg' when finding the command
- [x] If we find 'gpg', ensure the version is 2 or higher
- [x] Add tests for version detection.