fix: Convert Devise.sign_out_via to String

Since lastest Alchemy main (8.1.0.a) this needs
to be a String and cannot be a Symbol.

Author: tvdeyen

config/initializers/alchemy.rb

@@ -38,10 +38,8 @@
   Alchemy.user_class_name = "Spree::User"
   Alchemy.current_user_method = :spree_current_user
 
-  if Alchemy.respond_to?(:logout_method)
-    Rails.application.config.after_initialize do
-      Alchemy.logout_method = Devise.sign_out_via
-    end
+  Rails.application.config.after_initialize do
+    Alchemy.logout_method = Devise.sign_out_via.to_s
   end
 end