Make it so that the import works for older versions of kombu

The import location for message was different in older versions
of kombu, so when using it for mocking we need to take into
account the older location so that the test correctly works out
as it should (without import errors).

Fixes bug 1376330

Change-Id: Ifad489f0e15490168f993f48bfedfd553acd4a69