Update to a newer hacking library
Update to a newer hacking library

The older hacking library has a cap on pbr <2.0, with the recent 2.0.0
release of PBR it's causing failures in the pep8 job. hacking isn't
kept in sync via the typical proposal-bot updates. Do it manually to
clear the gate issue.

Change-Id: I752f518611add90dd391982cb7dade9b599ff9d3
Related-Bug: #1668848

openstack-dev/pbr,openstack-dev/pbr,openstack-dev/pbr