Add pbr to all tox environments
Add pbr to all tox environments

Doc builds are failing, apparently it's fixed by having pbr installed.
PBR really should be installed in all tox environments so just put it in
the main tox deps.

Change-Id: Iffc9a343d2953800a0afb4a92a5a6835e868994d

stackforge/requests-mock,jamielennox/requests-mock