title: Airflow HttpSensor won't work
body: <p>I'm trying to create a HttpSensor in Airflow using the following code:</p>
  
  <pre><code>wait_to_launch = HttpSensor(
      task_id="wait-to-launch",
      endpoint='http://' + socket.gethostname() + ":8500/v1/kv/launch-cluster?raw",
      response_check=lambda response: True if 'oui'==response.content else False,
      dag=dag
  )
  </code></pre>
  
  <p>But I keep getting this error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "http_sensor_test.py", line 30, in &lt;module&gt;
      dag=dag
    File "/home/me/.local/lib/python2.7/site-packages/airflow/utils/decorators.py", line 86, in wrapper
      result = func(*args, **kwargs)
    File "/home/me/.local/lib/python2.7/site-packages/airflow/operators/sensors.py", line 663, in __init__
      self.hook = hooks.http_hook.HttpHook(method='GET', http_conn_id=http_conn_id)
    File "/home/me/.local/lib/python2.7/site-packages/airflow/utils/helpers.py", line 436, in __getattr__
      raise AttributeError
  AttributeError
  </code></pre>
  
  <p>What am I missing?</p>