Ubuntu 17.04 provides Docker 1.12.6, which has API version 1.24.
`dev_requirements.txt` specifies the docker-py 2.5.1, which by default
requests API version 1.30.

Hence when the SSH unit tests try to run the container specified in
`DockerizedSshDaemon` an error occurs

```
APIError: 400 Client Error: Bad Request ("client is newer than server
(client API version: 1.30, server API version: 1.24)")
```