Support for PRs from forks

[nomego]

When running this on a PR from another repo, check-runs exists with 403 and {"message":"Resource not accessible by integration","documentation_url":"https://developer.github.com/v3/checks/runs/#create-a-check-run"}

To fix this, we probably need to check if it's a PullRequestEvent and if event.repository.fork === true and then read something like event.pull_request.base.repo instead I think.