Upgrade flaky for multiprocessing nose support.

Flaky's latest release includes support for multiprocessing in nose. Upgrading in
order to further the end-goal of providing multiprocessing capability in our
bok-choy test suite.

requirements/edx/base.txt

@@ -128,7 +128,7 @@ diff-cover==0.8.0
 django-crum==0.5
 django_nose==1.4.1
 factory_boy==2.5.1
-flaky==2.0.3
+flaky==2.4.0
 freezegun==0.1.11
 mock-django==0.6.9
 mock==1.0.1