Use DOCKER_HOST environment variable to find Docker daemon

Removed all "smart" connection logic. Fig either uses the `DOCKER_HOST` environment variable if it's present, or passes `None` to docker-py, which does the "right thing" (i.e. falls back to the Unix socket).

This means we no longer know at URL-deciding time whether we can connect to the Docker daemon, so we wrap `dispatch` in a `try/except` which catches `requests.exceptions.ConnectionError`.