Update the metadata available for packages in repo.continuum.io.

        Returns a download worker.

def update_metadata(self):
        """
        Update the metadata available for packages in repo.continuum.io.

        Returns a download worker.
        """
        if self._data_directory is None:
            raise Exception('Need to call `api.set_data_directory` first.')

        metadata_url = 'https://repo.continuum.io/pkgs/metadata.json'
        filepath = os.sep.join([self._data_directory, 'metadata.json'])
        worker = self.download_requests(metadata_url, filepath)
        return worker