Use a more performant implementation of luhn check

```
>> require 'active_merchant'
=> true
>> require 'luhn_checksum'
=> true
>> class Luhner
>>   include ActiveMerchant::Billing::CreditCardMethods
>> end
=> Luhner
>> Benchmark.realtime { 1_000_000.times { LuhnChecksum.valid?("4111111111111111") } }
=> 3.122008
>> Benchmark.realtime { 1_000_000.times { Luhner.send(:valid_checksum?, "4111111111111111") } }
=> 6.562471
>> Benchmark.realtime { 1_000_000.times { LuhnChecksum.valid?("4111111111111111") } }
=> 2.980835
>> Benchmark.realtime { 1_000_000.times { Luhner.send(:valid_checksum?, "4111111111111111") } }
=> 6.498597
```