Replaces the SHA-pinned git dependency with the exact Hex pin, now
that encryptor 0.2.0 is published. Exact rather than a range: the
vault's stability notice reserves the right to change public APIs,
storage formats, and derivation constants between any pre-1.0
releases, so a range would let a host's deps.update change what
stored rows mean. This also unblocks mix hex.publish, which refuses
git dependencies.