Error after updating Peril

Using Heroku, we followed the steps to pull the latest from danger/peril@master and push to Heroku.

After the update, we're now getting an error:

```
Aug 15 18:28:42 peril-gatsbyjs app/web.1: info: ## issues.opened on unknown on gatsbyjs/peril-gatsbyjs
Aug 15 18:28:42 peril-gatsbyjs app/web.1: info: 2 runs needed: org/emptybody.ts, org/labeler.ts
Aug 15 18:28:43 peril-gatsbyjs heroku/router: at=info method=POST path="/webhook" host=peril-gatsbyjs.herokuapp.com request_id=10aeb0fa-bfd0-40fd-a8db-e5fcbbd66a6e fwd="192.30.253.29" dyno=web.1 connect=0ms service=981ms status=200 bytes=353 protocol=https
Aug 15 18:28:43 peril-gatsbyjs app/web.1: info: [runner] - Commenting, with results:
Aug 15 18:28:43 peril-gatsbyjs app/web.1: mds: 0
Aug 15 18:28:43 peril-gatsbyjs app/web.1: messages: 0
Aug 15 18:28:43 peril-gatsbyjs app/web.1: warns: 0
Aug 15 18:28:43 peril-gatsbyjs app/web.1: fails: 0
Aug 15 18:28:43 peril-gatsbyjs app/web.1: error: Error: TypeError: Cannot read property 'concat' of undefined
Aug 15 18:28:43 peril-gatsbyjs app/web.1: at Executor. (/app/node_modules/danger/distribution/runner/Executor.js:273:46)
```

Looking at the danger source, the `results.fails` property appears to be coming back undefined.

We also might be doing something wrong; our Peril instance recently started triple-firing and I have no idea how or why. Any insights here are much appreciated.

- Repo: https://github.com/gatsbyjs/peril-gatsbyjs
- Triple-tap: https://github.com/gatsbyjs/peril-gatsbyjs/issues/28
- Peril settings: https://github.com/gatsbyjs/peril-gatsbyjs/blob/72dca531d57b801c1c9c2520c0e0ba30d51d35bf/peril.settings.json

Thanks!