Revert "coveralls complains about a `pass` in DummyConnection, use `return` instead."

This reverts commit e6d183b1.

persistent/tests/test_picklecache.py

@@ -1136,7 +1136,7 @@ class CPickleCacheTests(PickleCacheTests):
 
         class DummyConnection(object):
           def register(self, obj):
-            return
+            pass
 
         dummy_connection = DummyConnection()