Use the KomojuGateway direct from activemerchant, rather than including its own version.

[robbie-mcfarlane]

No description provided.

[robbie-mcfarlane]

Ideally, the spree_komoju gem would use the KomojuGateway direct from activemerchant, rather than using a copy of it in this gem. This would allow the spree_komoju gem to use the latest version of the KomojuGateway which has been updated to use the new komoju testing server (https://komoju.com/api/v1), and dropped support for the old sandbox server (https://sandbox.komoju.com/api/v1).