Make compatible with new GitHub API

[mhkeller]

It looks like GitHub is changing the way they deliver webhook payloads

EDIT, better link to the post
https://developer.github.com/changes/2014-10-21-deployment-webhook-payload-changes/

[mhkeller]

This seems to be working after I patched it for the ping event #10

[antoineco]

Would it be possible to merge this patch? Right now Hookshot is broken unfortunately.

[mhkeller]

If there's still no movement on this, I'll put my fork with the fix onto npm.