title: Can not run DAGs since upgraded from Airflow 1.7
body: <p>We installed Airflow 1.7, and used it for several months.  I used PIP to uninstall ariflow 1.7 and install 1.9 (gory details are [here|<a href="https://stackoverflow.com/questions/49544320/airflow-initdb-failed-importerror-no-module-named-log-logging-mixin).]">airflow initdb failed: ImportError: No module named log.logging_mixin</a></p>
  
  <p>Since then, I haven't had a single DAG run.  I renamed and moved log files to match the 1.9 expectations, but still nothing happens.</p>
  
  <p>I have a "run every 40 minutes" DAG, it hasn't run since 3/28.  When I manually trigger it, no log file is created, nothing happens except I get a running DAG listed under "DAG Runs" (I do NOT get anything listed under "Recent Tasks", and "Last Run" does not get updated.</p>
  
  <p>I have a "Run once" DAG that I created.  I triggered it, same behavior.</p>
  
  <p>I have also tried running the example_bash_operator DAG.  Same behavior.</p>