docker-compose.yml: fix mongo version (3.4)
docker-compose.yml: fix mongo version (3.4)

there are a couple tags we could fix on -
'3.4.1', '3.4', '3' and 'latest' all point to the same image.

decided to fix on the minor version - this way we'll
automagically get patches for 3.4 as they come.

Issues: MEN-905

Signed-off-by: mchalczynski <3a897b56ed6fc284ae11e3e7ec3ea48d098a7178@rndity.com>

pasinskim/integration,GregorioDiStefano/integration,pasinskim/integration,GregorioDiStefano/integration,pasinskim/integration