Bump pbr to 2.0

pbr 1.10 is broken with Sphinx 1.5 [1]. We've somehow avoided any
issues, likely because we don't restrict ourselves to 1.10, but we
should make the version number explicit.

[1] https://bugs.launchpad.net/pbr/+bug/1379998

Change-Id: If3dbb6f2131530bcaddc96ca419ce56d2fb2d50a
Related-Bug: #1379998

test-requirements.txt

@@ -1,7 +1,7 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
-pbr>=1.6 # Apache-2.0
+pbr>=2.0 # Apache-2.0
 
 # Hacking already pins down pep8, pyflakes and flake8
 hacking>=0.12.0,!=0.13.0,<0.14  # Apache-2.0