Update configuration data if required.

        Notes:
          Per `the documentation`_, this updates the API configuration
          data *"every few days"*.

        .. _the documentation:
          http://docs.themoviedb.apiary.io/#reference/configuration

async def _update_config(self):
        """Update configuration data if required.

        Notes:
          Per `the documentation`_, this updates the API configuration
          data *"every few days"*.

        .. _the documentation:
          http://docs.themoviedb.apiary.io/#reference/configuration

        """
        if self.config['data'] is None or self.config_expired:
            data = await self.get_data(self.url_builder('configuration'))
            self.config = dict(data=data, last_update=datetime.now())