Switch from parallel to serial DAG

Using ploomber's serial task manager lets users actually follow progress; we
will probably want to configure this to be an option in the config yaml file
but for now I'm going to hardcode it as serial.

cupid/run.py

@@ -71,7 +71,7 @@ def run():
     #####################################################################
     # Ploomber - making a DAG
 
-    dag = ploomber.DAG(executor=ploomber.executors.Parallel())
+    dag = ploomber.DAG(executor=ploomber.executors.Serial())
 
 
     #####################################################################