listForks(cb) {
      return this._request('GET', `/repos/${this.__fullname}/forks`, null, cb);
   }
List a repository's forks
@see https://developer.github.com/v3/repos/forks/#list-forks
@param {Requestable.callback} cb - will receive the list of repositories forked from this one
@return {Promise} - the promise for the http request