support for relative paths in env_file

[brettmc]

services:
  my-service:
    env_file:
      - ./my.env

If an env_file contains a relative path (eg ./my.env), I receive the following error:

Traceback (most recent call last):
  File "/usr/bin/docker-compose-swarm-mode", line 9, in <module>
    load_entry_point('docker-compose-swarm-mode==2.1.0', 'console_scripts', 'docker-compose-swarm-mode')()
  File "/usr/lib/python2.7/site-packages/docker_compose_swarm_mode.py", line 518, in main
    getattr(docker_compose, args.command)()
  File "/usr/lib/python2.7/site-packages/docker_compose_swarm_mode.py", line 214, in up
    locals().get(parameter, unsupported)()
  File "/usr/lib/python2.7/site-packages/docker_compose_swarm_mode.py", line 205, in env_file
    with open(v) as env_file:
IOError: [Errno 2] No such file or directory: './my.env'

I think it must be looking in whichever directory the source is running out of?