The docker-py tests were broken, because the version of
py-test that was used, used a dependency that had a new
major release with a breaking change.

Unfortunately, it was not pinned to a specific version,
so when the dependency did the release, py-test broke;

```
22:16:47 Traceback (most recent call last):
22:16:47 File "/usr/local/bin/pytest", line 10, in <module>
22:16:47 sys.exit(main())
22:16:47 File "/usr/local/lib/python3.6/site-packages/_pytest/config/__init__.py", line 61, in main
22:16:47 config = _prepareconfig(args, plugins)
22:16:47 File "/usr/local/lib/python3.6/site-packages/_pytest/config/__init__.py", line 182, in _prepareconfig
22:16:47 config = get_config()
22:16:47 File "/usr/local/lib/python3.6/site-packages/_pytest/config/__init__.py", line 156, in get_config
22:16:47 pluginmanager.import_plugin(spec)
22:16:47 File "/usr/local/lib/python3.6/site-packages/_pytest/config/__init__.py", line 530, in import_plugin
22:16:47 __import__(importspec)
22:16:47 File "/usr/local/lib/python3.6/site-packages/_pytest/tmpdir.py", line 25, in <module>
22:16:47 class TempPathFactory(object):
22:16:47 File "/usr/local/lib/python3.6/site-packages/_pytest/tmpdir.py", line 35, in TempPathFactory
22:16:47 lambda p: Path(os.path.abspath(six.text_type(p)))
22:16:47 TypeError: attrib() got an unexpected keyword argument 'convert'
```

docker-py master has a fix for this (bumping the version of
`py-test`), but it's not in a release yet, and the docker cli that's used
in our CI is pinned to 17.06, which doesn't support building from a remote
git repository from a specific git commit.

To fix the immediate situation, this patch switches the docker-py
tests to run from the master branch.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>