faster heartbeat from celery morkers (#249)

docker/airflow/airflow.cfg

@@ -302,7 +302,8 @@ tls_key =
 # Task instances listen for external kill signal (when you clear tasks
 # from the CLI or the UI), this defines the frequency at which they should
 # listen (in seconds).
-job_heartbeat_sec = 60
+# From @max - I don't think tasks can't be picked up by workers faster than this!
+job_heartbeat_sec = 10
 
 # The scheduler constantly tries to trigger new tasks (look at the
 # scheduler section in the docs for more information). This defines