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
doing this conversion means we have to hand-update/redo all the logic when active-merchant updated

who about this dumb/low-maintenance version:
```Ruby
CARD_COMPANIES = {
 ....
}
regex = Regexp.union(CARD_COMPANIES.map(&:last))
```

should be pretty fast too since it's just 1 regex match / I'd doubt there would be any significant performance penalty.