Set min pbr version in setup_requires

This commit adds a minimum version on the setuptools setup_requires
for pbr. This is required to enable markers and also to ensure a
consistent version when installing with other packages.

Change-Id: I2168d91896ba82581929a2faeebace7419b4539e

setup.py

@@ -26,5 +26,5 @@ except ImportError:
     pass
 
 setuptools.setup(
-    setup_requires=['pbr'],
+    setup_requires=['pbr>=1.3'],
     pbr=True)