Automatically trigger `looker` Airflow DAG after merging looker/ related changes

[scholtzan]

To automatically deploy any changes to Looker, it would be good for CI to trigger the looker Airflow DAG when changes get merged in the looker/ directory.
This setup should be similar to https://github.com/mozilla/lookml-generator/blob/5257010d4d5696ff4b5fbb6969b590b47c1b01db/.circleci/config.yml#L216