celery fails to build in trusty (expired certificate)

[Matthias Klose]

seen in
https://launchpad.net/ubuntu/+archive/test-rebuild-20150317/+build/7073165

======================================================================
FAIL: test_has_expired (celery.tests.security.test_certificate.test_Certificate)
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Traceback (most recent call last):
  File "/build/buildd/celery-3.1.6/celery/tests/security/test_certificate.py", line 26, in test_has_expired
    self.assertFalse(Certificate(CERT1).has_expired())
AssertionError: 1L is not false

----------------------------------------------------------------------
Ran 1677 tests in 32.189s

FAILED (failures=1, skipped=44)
make[1]: *** [override_dh_auto_test] Error 1
make[1]: Leaving directory `/build/buildd/celery-3.1.6'