Update to modern pbr requirement

Old pbr did some funny things when you tried to install on repos without
tags. Bump the pbr dep so that we get deterministic behavior out of pbr
even without tags.

We should also consider making a tag or two anyways just to get
something under pbr as not everyone may have updated pbr.

Change-Id: I31c97cd81b4fdb75c96fa49d81ed1a757af1060f

requirements.txt

@@ -7,7 +7,7 @@ python-daemon<2.0 # BUg https://storyboard.openstack.org/#!/story/2000107
 irc!=11.1
 pyyaml
 lockfile
-pbr>=0.8.1,<1.0
+pbr>=1.6
 Babel>=0.9.6
 launchpadlib
 Jinja2

setup.py

@@ -18,5 +18,5 @@
 import setuptools
 
 setuptools.setup(
-    setup_requires=['pbr>=0.5.21,<1.0'],
+    setup_requires=['pbr>=1.8'],
     pbr=True)