stack/slapos: Version-up pbr otherwise can not install python-cliff.

I guess somewhere in python-cliff need to pin the version of the egg.

stack/slapos.cfg

@@ -127,7 +127,7 @@ lxml = 3.4.4
 meld3 = 1.0.2
 mr.developer = 1.33
 netaddr = 0.7.14
-pbr = 0.11.0
+pbr = 1.1.1
 prettytable = 0.7.2
 psutil = 3.0.0
 pyOpenSSL = 0.15.1