Fix `flake8` unit test.

Summary:
This unit test is failing for me. It seems that I have a newer version of `flake8`. We should probably provide support for the most recent versions of any external tools, at least until we can implement version-specific stuff.

I am running `2.1.0 (pep8: 1.5.6, pyflakes: 0.8.1, mccabe: 0.2.1) CPython 2.7.6 on Linux`.

Test Plan: `arc unit`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8987