require Kombu<=4.1.0

[willowj]

Kombu raise AttributeError(key)
AttributeError: async

  File "/usr/local/lib/python2.7/dist-packages/celery/worker/worker.py", line 241, in should_use_eventloop
    self._conninfo.transport.implements.async and
  File "/home/autossh/.local/lib/python2.7/site-packages/kombu/transport/base.py", line 125, in __getattr__
    raise AttributeError(key)
AttributeError: async

The latest version of Kombu renames async to asynchronous. celery-attributeerror-async-error

require Kombu<=4.1.0.