Update docker-py

Using commit:
https://github.com/aanand/docker-py/commit/b31bb4d879c8ecc37491edb9f56369c513577918
Update docker-py APIError imports
Fix regression when mounting volumes

Caused by
https://github.com/dotcloud/docker-py/commit/77fec67c608d0839a795f6eb807c9df2fd5bfd45