Update to celery 4
This is not backward compatible with celery 3, so pin celery>=4 See http://docs.celeryproject.org/en/4.0/whatsnew-4.0.html for summary of changes Change attributes from celery.app.config to lower case. Drop setting serializer to json in README and tests since json is now the default serializer. I had issues with test_workflow_group() where AsyncResult().get on the *group* task_id hang. It looks related to our test infrastructure and AFAIK there is no known usage of this for cubes using cubicweb-celerytask. I think we must dig into this bug but I already spent enough time on this, so let's move forward and skip this part of the test.