Travis webhook verification

https://github.com/abiosoft/caddy-git/blob/7c37f04cfac86475f954286aa1b7e7aba0fa1ca9/travis_hook.go#L81

Is the `handleSignature` function correct? The [Travis documentation](https://docs.travis-ci.com/user/notifications/#Verifying-Webhook-requests) seems to imply the code should check the `Signature` header against Travis’ public key.

It seems like the `Authorization` header is an [old method of authenticating Travis requests](https://blog.travis-ci.com/2016-08-31-webhook-delivery-changes) phased out in November 2016.