Fix verifyMessage return type definitions

[larabr]

verifyMessage does not return the same signatures object as openpgpjs.verify, since handleVerificationResult extracts the signature field from each entry:

• pmcrypto/lib/message/utils.js

  Line 164 in 55021f7

  .then((result) => handleVerificationResult(result, publicKeys, options.date))

• pmcrypto/lib/message/utils.js

  Line 101 in 55021f7

  signatures.push(await sigs[i].signature);

[twiss]

Superseded by #113