Acknowledge webhook creation ping

[mhkeller]

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.

[mhkeller]

Checking in on this.

[ScottONeal]

@mhkeller if you're still using this library, I've created a fork which supports Ping events and has unit tests.

Check it out at: https://www.npmjs.com/package/githooked

[mhkeller]

@ScottONeal awesome!