change Github auth header to X_HUB_SIGNATURE

Here is an example of headers sent by the webhook:
```
Request URL: https://bob.core-os.net/docker-build/github
Request method: POST
content-type: application/json
Expect: 
User-Agent: GitHub Hookshot d4a4685
X-GitHub-Delivery: ad9f97fc-02e5-11e4-977c-8ba4ce2d4c8b
X-GitHub-Event: ping
X-Hub-Signature: sha1=f619462e0400fb51cc0f81841c1bfa8c928184ca
```