Put pbr and six first in requirements list

Each of them have scenarios where it's important that they install
first.

Change-Id: Ia9fff6e94651693f9fa74ca676771b19144bcd8b

Author: emonty

requirements.txt

@@ -1,16 +1,17 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
+pbr>=0.6,!=0.7,<1.0
+six>=1.7.0
+
 Babel>=1.3
 cliff>=1.7.0  # Apache-2.0
 oslo.i18n>=1.0.0  # Apache-2.0
 oslo.utils>=1.0.0                       # Apache-2.0
-pbr>=0.6,!=0.7,<1.0
 python-glanceclient>=0.14.0
 python-keystoneclient>=0.11.1
 python-novaclient>=2.18.0
 python-cinderclient>=1.1.0
 python-neutronclient>=2.3.6,<3
 requests>=2.2.0,!=2.4.0
-six>=1.7.0
 stevedore>=1.0.0  # Apache-2.0