some changes regarding using my repo

* i set a custom domain to my app. so delete it would remove the custom domain. so use update instead
* my repo is in an organization account. so the pusher.email is `undefined` , so I have to add `undefined` into whitelist
* validate github ip for more security

I already submitted above PRs into anoopsinha/master but he doesn't seem to be maintaining that repo any more. So I wish these changed could be merged into your fork.