Update makefile commands to use exec instead of shell

This is more what people expect -- to run commands in an existing container

Makefile

@@ -20,10 +20,10 @@ migrate:
 	docker-compose run web python manage.py migrate --run-syncdb
 
 shell:
-	docker-compose run web bash
+	docker-compose exec web bash
 
 django-shell:
-	docker-compose run web ./manage.py shell
+	docker-compose exec web ./manage.py shell
 
 presto-cli:
 	docker-compose exec presto presto-cli