Fix clear_all_connections! NoMethodError

This fixes the following error when running Action Cable tests:

  ```
  NoMethodError: undefined method `clear_all_connections!' for #<ActiveRecord::ConnectionAdapters::PostgreSQLAdapter
  actioncable/test/subscription_adapter/postgresql_test.rb:38:in `teardown'
  ```

Author: cpb

actioncable/test/subscription_adapter/postgresql_test.rb

@@ -35,7 +35,7 @@ def setup
   def teardown
     super
 
-    ActiveRecord::Base.connection.clear_all_connections!
+    ActiveRecord::Base.connection_handler.clear_all_connections!
   end
 
   def cable_config