Update docker-py to 2.6.0 from 1.5.0

[kokosing]

When using current docker version and old docker-py it is possible to get some weird errors. I am getting this one:

"Cannot link to a non running container

Here someone suggest that we sholuld use docker network instead of link.

Update would be nice, but it is hard to tell how much changes are needed to update this lib.

[petroav]

Are you getting the error while running the product tests?

I feel like as long as the two versions are API compatible everything should work as expected. Did you try a Travis run with the new version?

[kokosing]

Yes, I got this error while running product tests. I haven't yet started to do anything with the new version.

[kokosing]

Fixed with #353