public function authenticationResponse()
    {
        $client = new Client();

        try {
            $response = $client->request('GET', static::AUTH_URL, [
                'headers' => [
                    'X-Auth-User' => $this->username,
                    'X-Auth-Key' => $this->password,
                ],
            ]);
        } catch (RequestException $e) {
            throw new AuthenticationFailedException('Given credentials are wrong.', 403);
        }

        return $response;
    }