Fix Valid Toggle

Fix `validToggler()` to correctly toggle on Expiry Date. Seems like
`jquery.payment` couldn't work with formatted date, as parsed by `val`.
I might be stupid, but this is the only way I got it to run.
In general, I think that this section of code can be tightened up significantly — a lot of the logic around toggling `card-valid` and `card-invalid` is duplicated. Perhaps add a new `cardValidToggler` for all the bound values that encapsulates this logic?