Don't warn about shouldComponentUpdate from @observer'd superclass

[kmdavis]

This pull fixes the issue where the following is logged erroneously:

Use shouldComponentUpdate in an observer based component breaks the behavior of observer and might lead to unexpected results. Manually implementing sCU should not be needed when using mobx-react.

This warning is misleading and incorrect if the shouldComponentUpdate that is being complained about was added by @observer itself. e.g.

@observer
class GenericComponent extends Component {}

@observer // should not complain, but it does
class SpecificComponent extends GenericComponent {}

[mweststrate]

Duplicate of #492, thanks for the effort though :-)