Fix clear_reloadable_connections! deprecation warning

This fixes the following warning when running Action Cable tests:

  ```
  DEPRECATION WARNING: Calling `ActiveRecord::Base.clear_reloadable_connections! is deprecated. Please call the method directly on the connection handler; for example: `ActiveRecord::Base.connection_handler.clear_reloadable_connections!`.
  ```

Author: cpb

actioncable/test/subscription_adapter/postgresql_test.rb

@@ -60,7 +60,7 @@ def active?
       assert_equal "hello world", queue.pop
     end
 
-    ActiveRecord::Base.clear_reloadable_connections!
+    ActiveRecord::Base.connection_handler.clear_reloadable_connections!
 
     assert adapter.active?
   end