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 #50 from spotify/rohan/travis

travis: Export MAVEN_OPTS in before_script
Merge pull request #47 from spotify/dano/curator-2.5.0

zk: use curator 2.5.0
prepare for sonatype hosting

* Use sonatype parent pom
* Add sonatype required elements
Merge pull request #48 from spotify/dano/sonatype

prepare for sonatype hosting