CohortMembership Race Condition Test

By using the before_after library, we can force a race condition to reliably
occur in the CohortMembership.save() method. This unit test will do just that,
and ensure that our race-condition-handling code functions as it should.

requirements/edx/base.txt

@@ -120,6 +120,7 @@ django_debug_toolbar==1.3.2
 
 # Used for testing
 astroid==1.3.8
+before_after==0.1.3
 bok-choy==0.4.7
 chrono==1.0.2
 coverage==4.0