Updated zake now that zake 0.0.20 is released

The newer zake fixes up watcher usage and makes those work
correctly so that integrating projects can correctly test
that watches are triggered in the correct manner.

This makes it more useful for taskflow & tooz & others that
use this library for testing zookeeper interactions (without
requiring a full zookeeper setup/deployment).

Change-Id: I8973ab25c7b5ad40cf3152df1b43da979ec3c730
Merge "Updated zake now that zake 0.0.20 is released"