Object oriented solution to this coding challenge.
Run tests:
ruby test/credit_card_test.rb
Examples:
credit_card = CreditCard.new('4929735477250543')
credit_card.valid? # => true
credit_card = CreditCard.new('5541801923795240')
credit_card.valid? # => false
# works with American Express
amex_card = CreditCard.new('342804633855673')
amex_card.valid? # => true