is_different behavior differs with numpy vs without numpy

[thequilo]

In #928 the issue came up that the behavior of is_different in custom_containers differs if numpy is installed vs when it is not installed. Here is an example:

>>> import sacred

>>> sacred.config.custom_containers.is_different([1, [1,2]], [1, [1,2]])
True

>>> sacred.optional.has_numpy = False

>>> sacred.config.custom_containers.is_different([1, [1,2]], [1, [1,2]])
False

This looks like undesired behavior, but I have to investigate further what impact this issue has before I fix it.