title: Airflow install broken. `table dag_stats already exists`
body: <p>My Airflow install has stopped working. UI Webserver doesn't come up. I think the problem is related to the following error that appears when I run</p>
  <p><code>&gt; airflow initdb</code></p>
  <pre><code>sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) table dag_stats already exists
  [SQL: 
  CREATE TABLE dag_stats (
      dag_id VARCHAR(250) NOT NULL, 
      state VARCHAR(50) NOT NULL, 
      count INTEGER NOT NULL, 
      dirty BOOLEAN NOT NULL, 
      PRIMARY KEY (dag_id, state), 
      CHECK (dirty IN (0, 1))
  )
  
  ]
  </code></pre>
  <p>Does anyone know how to fix this so I can get back to work?</p>