Revert to Airflow 1.8.1 (#57)
Revert to Airflow 1.8.1 (#57)

- Impacts our production Airflow systems.
- Should be related to https://issues.apache.org/jira/browse/AIRFLOW-1641 and https://stackoverflow.com/questions/45853013/airflow-tasks-get-stuck-at-queued-status-and-never-gets-running
datagovsg/airflow-pipeline,datagovsg/airflow-pipeline