Incorrect handling of credential expiry in github.com/nats-io/jwt

The AccountClaims.IsRevoked and Export.IsRevoked functions improperly validate expired credentials using the current system time rather than the issue time of the JWT to be tested.

These functions cannot be used properly. Newer versions of the jwt package provide an IsClaimRevoked method which performs correct validation. In these versions, the IsRevoked method always return true.