Issue: Use SigningCertUrl instead of SigningCertURL

[dlcron]

The response format has changed. There is no SigningCertURL

class SigningCertURLValidator(object):
    """
    Validate a message's SigningCertURL is in the expected format.
    """
    [..]

    def validate(self, message):
        if not isinstance(message, dict):
            raise ValidationError("Unexpected message type {!r}".format(type(message).__name__))

        url = message.get("SigningCertURL")

> obj['Sns'].keys()
dict_keys(['Type', 'MessageId', 'TopicArn', 'Subject', 'Message', 'Timestamp', 'SignatureVersion', 'Signature', 'SigningCertUrl', 'UnsubscribeUrl', 'MessageAttributes'])

> 'SigningCertUrl' in obj['Sns'].keys(), 'SigningCertURL' in obj['Sns'].keys()
(True, False)