help users avoid problems using different versions of gnupg with git-secret repos

[joshrabinowitz]

As per #760, we could help users avoid issues when mixing different versions of gnupg on the same repo.

@sobolevn suggests writing a '.gitsecret/VERSION file with gpg version', and @FanchenBao adds that 'we can update the VERSION file each time user run git secret hide, and verify against VERSION each time user run git secret cat or git secret reveal.'

Edit by @joshrabinowitz : I don't think only verifying on cat and reveal is sufficient, other commands can cause potential interoperability issues between different versions of gnupg

Edit: See also #204