[Fix gate]Update test requirement
[Fix gate]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: #1668848

Change-Id: I2fbdd6f472704d5e1c6f8cf1c2ac5b09ea05aead

openstack/oslo.context