Use the ActiveMerchant company specific credit card regexes

@ggrossman 

This makes it so we use Shipify's beautiful activemerchant 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
My only concern is that ActiveMerchant has a lot of dependencies that are never going to be used. Should we not worry about this? 
https://github.com/zendesk/credit_card_sanitizer/commit/d94f8c8831d6a5294958c5eae4c12e7d1906d6d8 <- that really sucks