def check_versions(self, conn):
        """
        :param conn: a DB API 2 connection
        :returns: a message with the versions that will be applied or None
        """
        scripts = self.read_scripts(skip_versions=self.get_db_versions(conn))
        versions = [s['version'] for s in scripts]
        if versions:
            return ('Your database is not updated. You can update it by '
                    'running oq engine --upgrade-db which will process the '
                    'following new versions: %s' % versions)
:param conn: a DB API 2 connection
        :returns: a message with the versions that will be applied or None