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
Definitely, Victor's transformed regex's create a maintenance issue; I figured it was OK since credit card prefixes don't change very often. But when they do, we're probably going to miss it.

Maybe the Grosser approach wins purely on the basis of Less Code Is Better? I