title: Airflow scheduler is not picking up job
body: <p>I've created a new Dag with following arguments:</p>
  
  <pre><code>default_args = {
      'owner': 'airflow',
      'depends_on_past': False,
      'start_date': datetime.now(),
      'email': ['airflow@example.com'],
      'email_on_failure': False,
      'email_on_retry': False,
      'retries': 1,
      'retry_delay': timedelta(minutes=5),
      'catchup': False,
      # 'queue': 'bash_queue',
      # 'pool': 'backfill',
      # 'priority_weight': 10,
      # 'end_date': datetime(2016, 1, 1),
      # 'wait_for_downstream': False,
      # 'dag': dag,
      # 'adhoc':False,
      # 'sla': timedelta(hours=2),
      # 'execution_timeout': timedelta(seconds=300),
      # 'on_failure_callback': some_function,
      # 'on_success_callback': some_other_function,
      # 'on_retry_callback': another_function,
      # 'trigger_rule': u'all_success'
  }
  
  dag = DAG(
      'sample_dag',
      default_args=default_args,
      description='sample dag',
      schedule_interval="44 * * * *")
  </code></pre>
  
  <p>But the scheduler is not picking up the dag when the time comes. And it is running fine when I manually trigger it. Is there anything I'm missing here?</p>
  
  <p>Also, the scheduler was throwing an error when the cron expression was <code>"*/5 * * * *"</code></p>
  
  <pre><code>CroniterBadCronError: Exactly 5 or 6 columns has to be specified for iteratorexpression.
  </code></pre>
  
  <p>But the cron expression looks good me.</p>