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`.
Merge pull request #19 from orchardup/use-docker-host-variable

Use DOCKER_HOST environment variable to find Docker daemon