Add support for latest flower with celery 5 (#163)

From version 1.0.1, Flower uses Celery 5 and has to be invoked in the same style as celery commands do.

Author: jakemanger

{{cookiecutter.project_slug}}/docker-compose.yml

@@ -34,7 +34,7 @@ services:
 
   flower:  
     image: mher/flower
-    command: flower --broker=redis://redis:6379/0 --port=5555
+    command: celery flower --broker=redis://redis:6379/0 --port=5555
     ports:  
         - 5555:5555
     depends_on: