zk: use curator 2.5.0

Curator 2.5.0 contains a PersistentEphemeralNode fix the we might have
been hitting when seeing ephemeral node split brain syndrome.

https://github.com/apache/curator/commit/bfdef2ddc1c3f2121256045690b39baafa269151
Merge pull request #47 from spotify/dano/curator-2.5.0

zk: use curator 2.5.0