refactor evo ca gateway to improve readability

I was just reading through the code and found a couple areas that could be extracted out to make the Evo Gateway a little more readable. The duplication was found from Code Climate: https://codeclimate.com/github/Shopify/active_merchant/ActiveMerchant::Billing::EvoCaGateway?from_sha=5e7faae7&to_sha=64e864d9