Let's get with the times.

* Use "docker compose" instead of "docker-compose".

* Remove the 'version' key from exp/docker-compose*.yml files because
  newer versions of the docker compose plugin generate the following
  warning:

  WARN[0000] .../train-dev/exp/docker-compose.yml: `version` is obsolete
  WARN[0000] .../train-dev/exp/docker-compose-override-for-bind-mounts.yml: `version` is obsolete

Change-Id: I8961b949649d7338d8e398cf44735d147be73502