Set application_id attribute to Solidus
Set application_id attribute to Solidus

In order to track payments processed by Solidus stores, Braintree
requires us to pass the :channel parameter with the payment. Right now
ActiveMerchant is setting that using the application_id class attribute
on the gateway here:

  https://github.com/activemerchant/active_merchant/blob/master/lib/active_merchant/billing/gateways/braintree_blue.rb#L575

solidusio/solidus_gateway,solidusio/solidus_gateway