Archived repositories should be excluded

Since [GitHub let archiving repos](https://github.com/blog/2460-archiving-repositories) (and GitLab too), dependabot should take care of [the _archived_ property](https://developer.github.com/v3/repos/#response-3) to:
* Avoid listing archived repo when creating a new dependabot project.
* Kill any attempt to bump dependencies on an archived repo.

🌈 