Allow extension to work with Solidus 2.2

prior to the new gateways design payment methods were inheriting
from Spree::Gateway and they needed to be instantiated by calling
`provider`. This commit makes it possible to support both versions.

It uses the new method added into 0.2.2 of solidus_support, this is
why the dependency has been updated.

app/models/solidus_affirm/checkout.rb

@@ -1,5 +1,5 @@
 module SolidusAffirm
-  class Checkout < ActiveRecord::Base
+  class Checkout < SolidusSupport.payment_source_parent_class
     self.table_name = "affirm_checkouts"
 
     def actions

app/models/solidus_affirm/gateway.rb

@@ -1,5 +1,5 @@
 module SolidusAffirm
-  class Gateway < ::Spree::PaymentMethod
+  class Gateway < SolidusSupport.payment_method_parent_class
     preference :public_api_key, :string
     preference :private_api_key, :string
     preference :javascript_url, :string
@@ -44,7 +44,7 @@ def auto_capture
     # If the transaction has not yet been captured, we can void the transaction.
     # Otherwise, we need to issue a refund.
     def cancel(charge_id, try_credit = true)
-      gateway
+      initialize_gateway
 
       begin
         transaction = ::Affirm::Charge.find(charge_id)
@@ -74,5 +74,12 @@ def try_void(payment)
     def voidable?(transaction)
       transaction.status == "authorized"
     end
+
+    def initialize_gateway
+      # We can just leave gateway after Solidus 2.2 EOL
+      return gateway if respond_to?(:gateway)
+
+      provider
+    end
   end
 end

solidus_affirm.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'active_model_serializers', '~> 0.10'
   s.add_dependency 'affirm-ruby', '1.0.2'
   s.add_dependency 'solidus', ['>= 2.0', '< 3']
-  s.add_dependency "solidus_support"
+  s.add_dependency "solidus_support", '>= 0.2.2'
 
   s.add_development_dependency 'capybara', '~> 2.18'
   s.add_development_dependency 'coffee-rails'