widen credit card fields in /credit-card.html

In Firefox on Mac OS X, these fields were not wide enough to completely contain their text when filled in. This widens the form fields so they have just enough space to contain all of the text, in Firefox on Mac OS X on my machine.

Note that the fields are still too narrow to display the full “MM” and “YYYY” placeholders; they are still cut off. I tried widening the fields so you could see the whole placeholders, but then the form looked weird when those fields had values filled in.
Merge pull request #1713 from roryokane/widen-credit-card-fields

Widen credit card fields in /credit-card.html