Upgrade pbr, which I think is something that the older sqlalchemy-mig…

…rate version was holding back.

requirements.txt

@@ -30,7 +30,7 @@ passlib==1.6.5
 paste==1.7.5.1
 pastedeploy==1.5.2        # via pastescript, pylons
 pastescript==2.0.2        # via pylons
-pbr==1.10.0               # via sqlalchemy-migrate
+pbr==5.1.3                # via sqlalchemy-migrate
 polib==1.0.7
 psycopg2==2.7.3.2
 pygments==2.2.0           # via weberror