Changed required for Cryptography Deprecation
Changed required for Cryptography Deprecation

Keylime was experiencing the following deprecation warning:

"CryptographyDeprecationWarning: signer and verifier have been
deprecated. Please use sign and verify instead."

This resolves the warning by updating to useing `verify` instead
of `verifier`

Resolves: #222

mit-ll/python-keylime,mit-ll/python-keylime,mit-ll/python-keylime,mit-ll/python-keylime