Update zake to requirements version

Zake has been updated in the global requirements
repository to be zake>=0.0.20, so in order for the
requirements bot to get its change through (without
updating hacking) we need to separate this change
out.

Change-Id: I5cf4407104409785fe4ee746af9fed7b766474e4

test-requirements.txt

@@ -5,7 +5,7 @@ mock>=1.0
 python-subunit>=0.0.18
 testrepository>=0.0.18
 testtools>=0.9.34
-zake>=0.0.18
+zake>=0.0.20 # Apache-2.0
 # docs build jobs
 sphinx>=1.2.1,<1.3
 oslosphinx