Adding a dependency to zookeeper collector

I've been struggling with this collector for quite some time, until I finally realized it's using the "mntr" zookeeper command, only available from zookeeper version 3.4.0
https://zookeeper.apache.org/doc/trunk/zookeeperAdmin.html#sc_zkCommands

I thought to put a small friendly reminder to the doc so the next person won't spend that much time.