unstar(cb) {
      return this._request('DELETE', `/gists/${this.__id}/star`, null, cb);
   }
Unstar a gist.
@see https://developer.github.com/v3/gists/#unstar-a-gist
@param {Requestable.callback} [cb] - will receive true if the request is successful
@return {Promise} - the Promise for the http request