add flaky to requirements file

requirements/edx/base.txt

@@ -136,6 +136,7 @@ diff-cover==0.7.3
 django-crum==0.5
 django_nose==1.3
 factory_boy==2.2.1
+flaky==1.1.0
 freezegun==0.1.11
 lettuce==0.2.20
 mock-django==0.6.6