Bump version of PBR.

PBR had an issue building sphinx autodocs, which was fixed between
0.5.11 and 0.5.12. Let's bump the minimum version so that docs build
properly!

Fixes bug 1186834

Change-Id: I069dcc3d170eefadc90206be2b3c46d605917cbb
Merge "Bump version of PBR."