Use the most recent version of jplock/zookeeper

The maintainer of the zookeeper image does not keep the 'latest' tag up
to date, so we have to manually request it.

See https://registry.hub.docker.com/u/jplock/zookeeper/

The particular reason for using the latest version is to work around
this bug in Docker 1.3: https://github.com/docker/docker/issues/9327
(The bug is triggered with 'jplock/zookeeper:latest' but not with
'jplock/zookeeper:3.4.6'.)