Strip the key and secret before using them for AmazonSNS

After hearing from a few folks in an [old issue](https://github.com/github/github-services/pull/416), I wanted to make this small change. Now, we'll strip the config value before using them to connect with Amazon. This seems to fix some pain people have felt using this service.

(FYI: I don't use this hook at all so I'd love some feedback from those who have used it. /cc @mr-empora @smillerk2)