Use the ActiveMerchent luhn_check and brand methods

@ggrossman 

This makes it so we use Shipify's beautiful activemerchent gem.

Should I add tests for the `brand?` method?
They are all here  https://github.com/Shopify/active_merchant/blob/master/test/unit/credit_card_methods_test.rb
Dunno, maybe I'd just use `valid_number?` to avoid having to sneak into their class privates