Closes #269: setup_requires pbr 1.3

less than 1.3 and the boolean environment marker expression is
reflected into setuptools incorrectly.
diff --git a/setup.py b/setup.py
index c72c231..1a18577 100755
--- a/setup.py
+++ b/setup.py
@@ -3,5 +3,5 @@
 
 
 setuptools.setup(
-    setup_requires=['pbr'],
+    setup_requires=['pbr>=1.3'],
     pbr=True)