Follow-up to #562 -- bug with Transform / Mobject.become

[behackl]

git bisect says that merging #562 introduced a problem with Mobject.become: The example

class FadeInAndOut(Scene):
    def construct(self):
        square = Square(color=BLUE).shift(2 * UP)
        annotation = Text("Fade In", height=0.8)
        self.add(annotation)
        self.play(FadeIn(square))

        annotation.become(Text("Fade Out", height=0.8))
        self.add(annotation)
        self.play(FadeOut(square))

from our documentation renders as

Same thing if the call to become is replaced by playing a Transform animation; the old text is simply kept on screen.

Originally posted by @behackl in #562 (comment)