Solve hacking/pbr requirements conflict

* requirements.txt: Add an empty line making it more clear that pbr
has to appear first in the requirements list, before other entries.
Also specify a newer required minimum pbr since the old one had
bugs and ought to be avoided.

* setup.py: Stop declaring the pbr version range here, since it's
handled separately by the requirements list.

* test-requirements.txt: Allow newer hacking with solvable pbr
second-order requirement and move it to the top of the list so that
it gets satisfied before other test requirements.

Change-Id: I4f80094c719f43a1261091263ccf60eefe5d9af1

requirements.txt

@@ -1,3 +1,4 @@
-pbr>=0.5.21,<1.0
+pbr>=0.6,<1.0
+
 extras
 python-daemon

setup.py

@@ -17,5 +17,5 @@
 import setuptools
 
 setuptools.setup(
-    setup_requires=['pbr>=0.5.21,<1.0'],
+    setup_requires=['pbr'],
     pbr=True)

test-requirements.txt

@@ -1,7 +1,8 @@
+hacking>=0.5.3,<0.8
+
 coverage>=3.6
 discover
 fixtures>=0.3.12
-hacking>=0.5.3,<0.6
 python-subunit
 statsd>=1.0.0,<3.0
 sphinx>=1.1.2,<1.2