Upgrade to Curator 2.2.0-incubating.

[Release notes] (https://issues.apache.org/jira/secure/ReleaseNote.jspa?projectId=12314425&version=12324635)

Note that [CURATOR-44] (https://issues.apache.org/jira/browse/CURATOR-44) only changed Javadoc and [CURATOR-46] (https://issues.apache.org/jira/browse/CURATOR-46) appears to have been lost and isn't actually part of the 2.2.0-incubator release.

That leaves [CURATOR-38] (https://issues.apache.org/jira/browse/CURATOR-38) as the only potentially backward-compatible change, but not one that affects curator-extensions since it doesn't depend on curator-x-discovery.