Last week’s episode was a surprise game-changer, merging the two agencies so that Don and Peggy (Jon Hamm, Elisabeth Moss) will be working together once more. And here comes another game-changer, I suspect: Mad Men has been tiptoeing, none too subtly, towards one of the most major and tragic events of the summer of 1968: the assassination of Robert F. Kennedy. It doesn’t necessarily happen tonight, but the events on Mad Men are so close, I’m betting it is. And it ought to chang