code: def check_versions(self, conn):
          
          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)
docs: :param conn: a DB API 2 connection
          :returns: a message with the versions that will be applied or None