fixed flake8 version

.travis.yml

@@ -18,7 +18,7 @@ install:
   - pip install --upgrade pip wheel
   - pip install --upgrade setuptools
   - pip install pip
-  - pip install flake8
+  - pip install flake8==3.3.0
   - pip install pyflakes==1.1.0
   - pip install coverage
   - pip install pytest