protected function sendSlackNotification($url, $payload) {
    $this->say("Sending slack notification...");
    $data = "payload=" . json_encode($payload);
    $ch = curl_init($url);
    curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
    curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
    $result = curl_exec($ch);
    curl_close($ch);
  }