This isn't quite a bug or a feature request, so I guess it goes here.
Could you maybe put a green check next to the option you're choosing when exchanging travellers (and maybe a red x on the unchosen option)?
As it is, it's not completely intuitive whether the white or black option is the one you selected. I clicked on keep, and wound up de-selecting it, thus delaying my exchange for a whole shuffle.
Anyway, just a minor thing. I love the implementation as a whole.