curator 2.3.0

In Netflix, we're not using curator incubating version anymore. Our platform based zookeeper client's dependency resolution is always overriding curator version and it's not easy to override with 2.1.0-incubating version. If you don't have any specific reason to stay with 2.1.0-incubating, please accept this pull request and release the new one for Netflix to enjoy Druid 0.6.

Thank you
Best, Jae