pipのversionが10.0.0~だとairflowのinstallでエラーになるためダウングレードする

[snagasawa]

Miserlou/Zappa#1471

 ---> Running in 46d234fc66a9
Collecting apache-airflow[celery,crypto,gcp_api,postgres]==1.8.2
  Downloading https://files.pythonhosted.org/packages/f1/83/a5fc6da0ced7c06f080b9b423b3483a926e7a387892a9013ec12229a0a89/apache-airflow-1.8.2.tar.gz (2.3MB)
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-install-3_6g48_h/apache-airflow/setup.py", line 302, in <module>
        do_setup()
      File "/tmp/pip-install-3_6g48_h/apache-airflow/setup.py", line 198, in do_setup
        check_previous()
      File "/tmp/pip-install-3_6g48_h/apache-airflow/setup.py", line 105, in check_previous
        in pip.get_installed_distributions()])
    AttributeError: module 'pip' has no attribute 'get_installed_distributions'

    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-install-3_6g48_h/apache-airflow/
The command '/bin/sh -c pip install apache-airflow[crypto,celery,postgres,gcp_api]==$AIRFLOW_VERSION' returned a non-zero code: 1
make: *** [build] Error 1