configure mergify.io

[johnynek]

This has been approved for this repo by our Twitter friends.

The goal is to automatically merge green PRs from scala-steward to keep things up to date.

We need to add a file like this:
https://github.com/scala-steward-org/scala-steward/blob/master/.mergify.yml

cc @regadas

[regadas]

Hi @johnynek! To be honest I'm a bit hesitant merging deps like that on a library, but looking at algebird's deps it might be ok. Let's give it a try.