getCollaborators(cb) {
      return this._request('GET', `/repos/${this.__fullname}/collaborators`, null, cb);
   }
List the users who are collaborators on the repository. The currently authenticated user must have
push access to use this method
@see https://developer.github.com/v3/repos/collaborators/#list-collaborators
@param {Requestable.callback} cb - will receive the list of collaborators
@return {Promise} - the promise for the http request