Acknowledge webhook creation ping

GitHub sends a ping even on Webhook creation: https://developer.github.com/webhooks/#ping-event

But since it doesn't follow the structure that `lib/index.js` is expecting, that script throws an error, such as when it can't find a string for `payload.ref`.

You could do fancier things to handle this such as emit a `ping` event but this fix will stop the script from breaking for now.