MerchantWarrior: Format expiration correctly

According to the MerchantWarrior API documentation, cardExpiryYear and
cardExpiryMonth must meet these conditions:

  cardExpiryMonth
  Example: 05
  Notes: This must be in MM format. The month must be zero-padded if
         it’s less than 10.
  Valid Length: 2 digits.

  cardExpiryYear
  Example: 13
  Notes: This must be in YY format.
  Valid Length: 2 digits.

Closes #665.