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
The gem dependency thing is kind of a bummer, but the list is a pretty decent intersection with a standard Rails app `activesupport`, `i18n`, `json`, `nokogiri`.

Maybe we can work with them to eliminate dependencies _within_ the gem, like activesupport does. Then we could require **one** file and get only what we need:

```ruby
require 'active_merchant/billing/credit_card_methods'
```

?