Disable SoftwarePlan.get_version cache in tests

A stab in the dark to fix TasksTest class cleanup failing during domain deletion with:
corehq.apps.accounting.models.FeatureRate.DoesNotExist: FeatureRate matching query does not exist.

Failure occurs when attempting to delete the "test" domain, but not "mirror".

corehq/apps/accounting/models.py

@@ -829,7 +829,7 @@ class SoftwarePlan(models.Model):
     class Meta(object):
         app_label = 'accounting'
 
-    @quickcache(vary_on=['self.pk'], timeout=10)
+    @quickcache(vary_on=['self.pk'], timeout=10, skip_arg=lambda *a, **k: settings.UNIT_TESTING)
     def get_version(self):
         try:
             return self.softwareplanversion_set.filter(is_active=True).latest('date_created')