Add renovatebot to automatically keep the dependencies up to date

[123Haynes]

I'd like to propose adding renovatebot to this repo.

The github app is free for open source projects: https://github.com/marketplace/renovate

It automatically creates a PR when a dependency update is available.
See renovatebot/renovate#3401 for an example of how a PR with a dependency update looks.