editLabel(label, labelData, cb) {
      return this._request('PATCH', `/repos/${this.__repository}/labels/${label}`, labelData, cb);
   }
Edit a label
@see https://developer.github.com/v3/issues/labels/#update-a-label
@param {string} label - the name of the label to edit
@param {Object} labelData - the updates to make to the label
@param {Requestable.callback} [cb] - will receive the updated label
@return {Promise} - the promise for the http request