payloadserver.go contains the
Payload function that gets deployed. To deploy
it from the command line, run:
$ gcloud functions deploy payloadserver --entry-point Payload --runtime go111 --trigger-http --set-env-vars HOOK_SECRET_KEY=<key>
Then point the Github webhook to it, and it should work.
To check logs from the command-line, run:
$ gcloud functions logs read payloadserver