Update README.md to contemplate unix sockets

[ecolabardini]

When installing Docker for Ubuntu 14.04 LTS (1) with apt, the docker daemon is using unix sockets. Therefore using something like

export DOCKER_HOST="tcp://127.0.0.1:2375"

will result in

$ DOCKER_RIAK_AUTOMATIC_CLUSTERING=1 DOCKER_RIAK_CLUSTER_SIZE=5 make start-cluster
./bin/start-cluster.sh
Cannot connect to the Docker daemon. Is the docker daemon running on this host?

Bringing up cluster nodes:

make: *** [start-cluster] Error 125`

For future installers would be great that the "Install Docker" section on the README.md contemplate Unix sockets, e.g.:

export DOCKER_HOST="unix:///var/run/docker.sock"

(1) https://docs.docker.com/engine/installation/linux/ubuntulinux/