Pin flake8 version

Signed-off-by: Joffrey F <joffrey@docker.com>

test-requirements.txt

@@ -2,4 +2,4 @@ mock==1.0.1
 pytest==2.9.1
 coverage==3.7.1
 pytest-cov==2.1.0
-flake8==2.4.1
+flake8==3.4.1

tox.ini

@@ -12,4 +12,5 @@ deps =
 
 [testenv:flake8]
 commands = flake8 docker tests setup.py
-deps = flake8
+deps =
+    -r{toxinidir}/test-requirements.txt