title: Error executing S3Hook list_keys or read_key methods
body: <p>I get this error message: 
  <code>{logging_mixin.py:112} INFO - [2020-03-22 12:34:53,672] {local_task_job.py:103} INFO - Task exited with return code -6
  </code> when I use the <code>list_keys</code> or <code>read_key</code> methods of S3 hook. The <code>get_credentials</code> method works fine though. Have searched around and can't find why this occurs.</p>
  
  <p>I'm using <code>apache-airflow==1.10.9</code>, <code>boto3==1.12.21</code>, <code>botocore==1.15.21</code></p>
  
  <p>Here's my code for my custom operator that makes use of the S3Hook:</p>
  
  <pre><code>class SASValueToRedshiftOperator(BaseOperator):
      """Custom Operator for extracting data from SAS source code.
      Attributes:
          ui_color (str): color code for task in Airflow UI.
      """
      ui_color = '#358150'
  
      @apply_defaults
      def __init__(self,
                   aws_credentials_id="",
                   redshift_conn_id="",
                   table="",
                   s3_bucket="",
                   s3_key="",
                   sas_value="",
                   columns="",
                   *args, **kwargs):
          """Extracts label mappings from SAS source code and store as Redshift table
          Args:
              aws_credentials_id (str): Airflow connection ID for AWS key and secret.
              redshift_conn_id (str): Airflow connection ID for redshift database.
              table (str): Name of table to load data to.
              s3_bucket (str): S3 Bucket Name Where SAS source code is store.
              s3_key (str): S3 Key Name for SAS source code.
              sas_value (str): value to search for in sas file for extraction of data.
              columns (list): resulting data column names.
          Returns:
              None
          """
          super(SASValueToRedshiftOperator, self).__init__(*args, **kwargs)
          self.aws_credentials_id = aws_credentials_id
          self.redshift_conn_id = redshift_conn_id
          self.table = table
          self.s3_bucket = s3_bucket
          self.s3_key = s3_key
          self.sas_value = sas_value
          self.columns = columns
  
      def execute(self, context):
          """Executes task for staging to redshift.
          Args:
              context (:obj:`dict`): Dict with values to apply on content.
          Returns:
              None   
          """
          s3 = S3Hook(self.aws_credentials_id)
          redshift_conn = BaseHook.get_connection(self.redshift_conn_id)
  
          self.log.info(s3)
          self.log.info(s3.get_credentials())
          self.log.info(s3.list_keys(self.s3_bucket))
  </code></pre>