Relax factory-boy requirement to >= 2.4.1

We don't need to specify exact version (2.5.2). Furthermore, we don't
even need factory-boy 2.5.x, the APIs used in factories.py can be
satisfied by factory-boy 2.4.1 that is available as a deb package in
Debian jessie and Ubuntu vivid.

Change-Id: I1e82ef3fc5b344d58497a4d4c396e4bc866e310c

test-requirements.txt

@@ -1,5 +1,5 @@
 mock >= 1.0.1
 pytest >= 2.7.1
 pytest-django >= 2.8.0
-factory_boy == 2.5.2
+factory_boy >= 2.4.1
 lxml >= 3.4.4