Remove version attribute from docker-compose.yaml

The version defined in docker-compose.yaml is ignored and raises a
warning

"WARN[0000] /xxx/gerrit-homepage/docker-compose.yml:
the attribute `version` is obsolete, it will be ignored, please remove
it to avoid potential confusion"

Remove the version attribute to fix this warning.

Change-Id: I48cd2a63fbc4f28c0269174a2e1d1ae55369d9d6