don't prevent default unless stripe comes into play

Further to 
https://github.com/drastik/civicrm_stripe/commit/659de5bf11c7685ed6758bc9dec5c6190fba5eb6

I found the preventDefault was still kicking in even when stripe was not meant to be invoked (e.g paylater).

I don't know if this is the best fix but moving the preventDefault & stopPropagation until after the early returns seems to help