Use docker-client 2.6.4

-RN
Use docker-client 2.6.4
Contains a fix for connection pooling:

> By default, the Apache HTTP client will only use two connections per
> server. Instead, we want to use all the available connections in the
> connection pool (20). This is to prevent issues where requests get blocked
> while waiting on Docker to pull an image, stop a container, etc.
Merge pull request #269 from spotify/rohan/fix-tests

Use docker-client 2.6.4