Avoid to use a method that acts on the hash

To avoid a deprecation warning.

activerecord/test/cases/database_configurations_test.rb

@@ -89,7 +89,7 @@ def test_each_is_deprecated
   end
 
   def test_first_is_deprecated
-    first_config = ActiveRecord::Base.configurations.values.first
+    first_config = ActiveRecord::Base.configurations.configurations.map(&:config).first
     assert_deprecated do
       env_name, config = ActiveRecord::Base.configurations.first
       assert_equal "arunit", env_name