Return early for GitHub ping payloads (avoids error message in log)

This eliminates the error message mentioned over in #1337. Specifically:

`github-commits error: TypeError: Cannot read property 'length' of undefined. Payload: {"zen":"It's not fully shipped until it's fast.","hook_id":1833128}`

The request works fine with or without this change, but this change eliminates the error from appearing in the Hubot logs.

(Fixes #1337)