It is possible to assign a task to the dag using the bitshift operators, however it doesn't pick up default_args when done this way <https://issues.apache.org/jira/browse/AIRFLOW-883>:
```
dag = DAG('my_dag', default_args=default_args)
dummy = DummyOperator(task_id='dummy')
dag >> dummy
```
We could fix that, but how about instead we remove this way of assigning tasks
to dags, leaving the context manager (`with dag:`) and other constructions
(`Operator(..., dag=dag)`)
Thoughts?
