Pin version of flake8 to 3.6.0 (current latest)

As things stand, tox can magically update flake8 on us and cause build
breakage.

tox.ini

@@ -16,6 +16,6 @@ commands = pytest {posargs}
 [testenv:flake8]
 basepython = python2.7
 deps =
-    flake8
+    flake8==3.6.0
 commands =
     flake8 mozetl tests --max-line-length=100