Don't return false on _.changeSlide

[alexparker]

I think the default case of the message switch used in the _.changeSlide method should not return false. It seems it would be fine in one of the other cases to call event.stopPropagation() if there is a cause or reason for preventing the bubbling, otherwise in the default case it should not return false.

https://github.com/kenwheeler/slick/blob/master/slick/slick.js#L572

This prevents any flexibility in binding any custom event handlers to the dot clicking interaction (ie pausing the autoplay once the user has clicked a slick dot)

Would you be willing to change this to just return perhaps?