MySQL doesn't work with foreign keys

This was previously fixed in e84799d9 but broken in 3f596f8a. This commit
reintroduced the conditional that prevents the foreign keys from being
added to MySQL databases.

activerecord/test/schema/schema.rb

@@ -83,7 +83,10 @@ def except(adapter_names_to_exclude)
 
   create_table :author_addresses, force: true do |t|
   end
-  add_foreign_key :authors, :author_addresses
+
+  unless current_adapter?(:MysqlAdapter, :Mysql2Adapter)
+    add_foreign_key :authors, :author_addresses
+  end
 
   create_table :author_favorites, force: true do |t|
     t.column :author_id, :integer