Use the ActiveMerchent company specific credit card regexes

@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
also: nobody knows if this is actually correct / if all conversions were done correctly (now or when we update the patterns), if we use a dumb approach and find a bug we can open it against activemerchant / fix it with confidence.