Update Celery to 4.1.1

[keckelt]

Creating the Celery Docker container with v4.0.1, the aysnc dependency was not found/installed.

WOrked fter updating to 4.1.1, probably due to it's deprecation: https://pypi.org/project/async/

[thinkh]

We updated to v4.2.2 with #27.