getRevision(revision, cb) {
      return this._request('GET', `/gists/${this.__id}/${revision}`, null, cb);
   }
Fetch one of the gist's revision.
@see https://developer.github.com/v3/gists/#get-a-specific-revision-of-a-gist
@param {string} revision - the id of the revision
@param {Requestable.callback} [cb] - will receive the revision
@return {Promise} - the Promise for the http request