deleteRef(ref, cb) {
      return this._request('DELETE', `/repos/${this.__fullname}/git/refs/${ref}`, null, cb);
   }
Delete a reference
@see https://developer.github.com/v3/git/refs/#delete-a-reference
@param {string} ref - the name of the ref to delte
@param {Requestable.callback} [cb] - will receive true if the request is successful
@return {Promise} - the promise for the http request