Update test requirement
Update test requirement

Since pbr already landed and the old version of hacking seems not
work very well with pbr>=2, we should update it to match global
requirement.
Partial-Bug: #1669276
Partial-Bug: #1668848

Change-Id: Id74bd427bbb7f1ffb364dd24ede18d920fe89c8c

openstack/python-heatclient