update(gist, cb) {
      return this._request('PATCH', `/gists/${this.__id}`, gist, cb);
   }
Update a gist.
@see https://developer.github.com/v3/gists/#edit-a-gist
@param {Object} gist - the new data for the gist
@param {Requestable.callback} [cb] - the function that receives the API result
@return {Promise} - the Promise for the http request