Writes lock file

        :param last_update: last update of app

def write_lock(self, last_update=datetime.datetime.now()):
        """Writes lock file

        :param last_update: last update of app
        """
        data = {
            "last_update": last_update.strftime(AppCronLock.DATETIME_FORMAT)
        }

        with open(self.lock_file, "w") as writer:
            json.dump(data, writer)