Deprecation warning for newer Express versions

```
GMT express deprecated res.send(status, body): 

Use res.status(status).send(body) instead at node_modules/hookshot/lib/index.js:33:9
```

Seems like an easy enough fix. I feel like your testing setup for webhooks is probably more ready-to-go than mine, but if you'd prefer a PR I can probably do one at some point.
