title: Airflow <> Slack
body: <p>I just started learning Airflow and I'm trying to create a small airflow script to send a notification to slack but somehow it fails to call for Slack API. First task works fine. Here is the code I use: </p>
  
  <pre><code>from airflow import DAG
  from airflow.contrib.operators.bigquery_check_operator import BigQueryCheckOperator
  from airflow.operators import BashOperator
  from slackclient import SlackClient
  from airflow.operators.slack_operator import SlackAPIPostOperator
  from datetime import datetime, timedelta
  
  default_args = {
      'owner': 'test',
      'depends_on_past': False,
      'start_date': datetime.today(),
      'email': ['test@test'],
      'email_on_failure': True,
      'email_on_retry': False,
      'retries': 4,
      'retry_delay': timedelta(minutes=5),
  }
  
  # Create the DAG
  dag = DAG('slack_test',\
      default_args=default_args,\
      schedule_interval='15 * * * *')
  
  final_check = BigQueryCheckOperator(
      task_id='bq_check_agg',
      sql='''
      #legacy sql
      SELECT
      max(date) as max_date
      FROM
      [test:test.test]
      WHERE date &lt;= current_timestamp()
      ''',
      dag = dag)
  
      # slack notification
  slack_notify = SlackAPIPostOperator(
      task_id='slack_notify',
      username  = 'Airflow',
      token='....',
      channel='#test-channel',
      text='ETL DONE!',
      dag=dag
  )
  slack_notify.set_upstream(final_check)
  </code></pre>
  
  <p>When I try to test <code>slack_notify</code> task, it fails. Here is the error I receive: </p>
  
  <pre><code>airflow.exceptions.AirflowException: Slack API call failed (%s)
  </code></pre>
  
  <p>Couldn't find any solutions for that online. Is something wrong with the slack authentication ? </p>