Update to docker-py 0.3.1

From https://github.com/dotcloud/docker-py/commit/7f55a101f813f3e96413d1b577e98d9467b0bffc

This now requires Docker 0.9 or greater.
Fix build output

docker-py now streams us the raw JSON events, so we have to replicate
the Docker client's progress logic.

On the bright side, we now have well-behaved progress bars when pulling
an image during `fig build` (no more ski slopes) and `fig up` (no more
silence).