Add link to original mask_modules code snippet (thanks runeh!)

celery/tests/utils.py

@@ -61,6 +61,9 @@ def skip_unless(predicate, reason):
     return skip_if(not predicate, reason)
 
 
+
+# Taken from
+# http://bitbucket.org/runeh/snippets/src/tip/missing_modules.py
 @contextmanager
 def mask_modules(*modnames):
     """Ban some modules from being importable inside the context