Fix for Flower 1.0+

[callmephilip]

As per the doc:

Important Please note that from version 1.0.1 Flower uses Celery 5 and has to be invoked in the same style as celery commands do.
The key takeaway here is that the Celery app’s arguments have to be specified after the celery command and Flower’s arguments have to be specified after the flower sub-command.

Which means Procfile should be:

web: celery --broker=$BROKER_URL flower --port=$PORT --basic_auth=$FLOWER_BASIC_AUTH