Unit testing: real GitHub request

We've caught a real request received by the Nasqueron notifications
center to use as a real data test.

It's available at payload.json.

X-GitHub-Delivery: c9105400-90e1-11e5-872f-8f3e950de755
X-GitHub-Event: ping
X-Hub-Signature: sha1=ab05a3aef13696b60a60a8064b9fda31a8c77582