follow(username, cb) {
      return this._request('PUT', `/user/following/${username}`, null, cb);
   }
Have the authenticated user follow this user
@see https://developer.github.com/v3/users/followers/#follow-a-user
@param {string} username - the user to follow
@param {Requestable.callback} [cb] - will receive true if the request succeeds
@return {Promise} - the promise for the http request