Add pbr to installation requirements

Add pbr to the list of installation requirements so that it is installed
via pip before this library is installed, instead of with easy_install.
This avoids issues like Bug #1384919, and ensures that projects that use
this library as a dependency are properly installed.

Change-Id: I6c155370dbd01fe4748d5137bdf288e8d3e1a67e
Imported Translations from Transifex

For more information about this automatic import see:
https://wiki.openstack.org/wiki/Translations/Infrastructure

Change-Id: I0700b92fc3f9b97e91961ebacb41420e2ab82164
Merge "Add pbr to installation requirements"