make pbr use hacking directly

Remove flake8 from pbr's requirements file, we should be requiring
hacking instead, and let hacking pull in flake8.

Change-Id: I6e7135a07e3ca6b9e26fcc9623b1434ecba1444f

test-requirements.txt

@@ -1,7 +1,7 @@
 coverage>=3.6
 discover
 fixtures>=0.3.14
-flake8==2.0
+hacking>=0.8,<0.9
 mock>=1.0
 python-subunit>=0.0.18
 sphinx>=1.1.2,<1.2

tox.ini

@@ -19,8 +19,7 @@ downloadcache = ~/cache/pip
 [testenv:pep8]
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-       hacking
-commands = flake8
+commands = flake8 {posargs}
 
 [testenv:cover]
 setenv = VIRTUAL_ENV={envdir}