Use default Spree::Event::Adapter

On installing this version (currenlty unreleased) of Solidus, we
no longer support the ActiveSupport Notification adaptor, but rather
the new `Spree::Event::Adapters::Default`

This commit add that configuration to the spree.rb template so every
generated install is using the new defaults.

core/lib/generators/solidus/install/templates/config/initializers/spree.rb.tt

@@ -1,3 +1,5 @@
+require "spree/event/adapters/default"
+
 # Configure Solidus Preferences
 # See http://docs.solidus.io/Spree/AppConfiguration.html for details
 
@@ -31,6 +33,8 @@ Spree.config do |config|
   # to a custom users role:
   # config.roles.assign_permissions :role_name, ['Spree::PermissionSets::CustomPermissionSet']
 
+  # Default event adapter:
+  config.events.adapter = Spree::Event::Adapters::Default.new
 
   # Frontend: