Without this version, markers will fail to install in a growing number
of projects.
Change global-requirements to prevent
pip -r requirements.txt; pip install .
failing with an already installed and too old for setup_requires pbr.
Change update.py to signal to setuptools the minimum version required,
giving clearer error messages in the case of downgrades during
devstack.
Change-Id: I074274b470a7a4f2c95211677b135038ab2c942c
0937f0d3a2