Github hooks appear to not be deleting when a trigger is deleted

Recently I was running the UI tests and one of the tests started failing. It was trying to create a Github trigger with the "push" event. I kept getting a 502 error back.

I tried another account and everything worked fine. I then tried to use the "delete" event and it worked.

So I was told to run a curl command against the hooks endpoint to see what hooks existed against that particular account and I saw a bunch with the "push" event and the one I created with the delete event.

I then tried to do a `wsk trigger delete` of the trigger with the "delete" event and when I inspected that hooks endpoint again I still saw the hook for it present.

I ran `wsk activation get ##` against the activation id that was returned in the `wsk trigger delete` command and saw the following:
```
"result": {
"error": "Found no existing webhooks for trigger URL https://9aac5786-5cd8-4eed-bde3-8d0b202ccb89:XAzJEaB0ijE1j65v9CKaCzAJoem1w6GtBGvMRrO1ri2cfEUyrfyU1Wka0zIrtPoY@openwhisk.ng.bluemix.net/api/v1/namespaces/eweiter%40us.ibm.com_dev/triggers/myGitTrigger"
}
```