Upgrade docker-client dependency

-RN

* This new docker client adds SSL/TLS support for Helios to Docker
  communications.
* The DockerCertificates class can now be used to build an SSL context for
  communicating with Docker over HTTPS with client/server authentication.
* Note that we ignore hostnames when verifying SSL certificates. Instead,
  we just make sure that the certificate is from the correct CA. Hostname
  verification is pointless here since everyone is just using self-signed
  certs anyway.
* The way we test this stuff is a bit interesting too: we start a
  container and run an HTTPS Docker instance inside the container,
  and see if we can connect to that with the appropriate certificates.

* Don't send requests to start a container to Docker using chunked
  encoding.
* This is to workaround a bug present in Docker 1.2.0, wherein the
  HostConfig is completely ignored if sent with chunked encoding.
* To enable testing this, we add ContainerInfo.hostConfig(), which returns
  the HostConfig we get from inspecting the container.
Default to Unix socket on Linux machines

On Linux machines, default to unix:///var/run/docker.sock to talk to Docker
instead of HTTP. This will help Helios work right out of the box on Linux
boxes.