support values of DOCKER_HOST with no explicit protocol

When the `DOCKER_HOST` environment variable does not specify a protocol, `docker` defaults to `"tcp://"` but `fig` dies without saying anything useful. This is very confusing to new users who might expect `fig` to be able to connect under the same circumstances as `docker`.

    $ echo $DOCKER_HOST
    192.168.59.103:2376

    $ docker version
    Client version: 1.3.0
    Client API version: 1.15
    Go version (client): go1.3.3
    Git commit (client): c78088f
    OS/Arch (client): darwin/amd64
    Server version: 1.3.0
    Server API version: 1.15
    Go version (server): go1.3.3
    Git commit (server): c78088f

    $ fig ps
    Traceback (most recent call last):
      File "/usr/local/bin/fig", line 9, in <module>
        load_entry_point('fig==1.0.1', 'console_scripts', 'fig')()
      File "/Library/Python/2.7/site-packages/fig/cli/main.py", line 31, in main
        command.sys_dispatch()
      File "/Library/Python/2.7/site-packages/fig/cli/docopt_command.py", line 21, in sys_dispatch
        self.dispatch(sys.argv[1:], None)
      File "/Library/Python/2.7/site-packages/fig/cli/command.py", line 28, in dispatch
        super(Command, self).dispatch(*args, **kwargs)
      File "/Library/Python/2.7/site-packages/fig/cli/docopt_command.py", line 24, in dispatch
        self.perform_command(*self.parse(argv, global_options))
      File "/Library/Python/2.7/site-packages/fig/cli/command.py", line 54, in perform_command
        verbose=options.get('--verbose'))
      File "/Library/Python/2.7/site-packages/fig/cli/command.py", line 83, in get_project
        self.get_client(verbose=verbose))
      File "/Library/Python/2.7/site-packages/fig/cli/command.py", line 59, in get_client
        client = docker_client()
      File "/Library/Python/2.7/site-packages/fig/cli/docker_client.py", line 21, in docker_client
        base_url = '%s://%s' % ('https', parts[1])
    IndexError: list index out of range

I propose changing the url parsing in `docker_client.py` to support this format without crashing.