American Express cards don't work with the U1 music store

[Joshua Hoover]

Steps to reproduce:

1. Add a track to your cart and checkout at https://one.ubuntu.com/music-store

2. Select to pay with American Express, enter all the details, including the 4-digit validation code into the "Validation number" field

Result: Invalid card details

Expected result: Payment goes through

NOTE: AmEx works on the music store in Rhythmbox and Banshee according to users who've tried both the web store and the store in the player.