Fix rake/notes_test

Presumably due to https://github.com/rails/sprockets-rails/pull/265 sprockets was trying to load the "scss" gem but it isn't in the gemfile:

```
ApplicationTests::RakeTests::RakeNotesTest#test_register_a_new_extension:
LoadError: cannot load such file -- sass
```

If we use an empty precompile list, it won't try to load sass.

railties/test/application/rake/notes_test.rb

@@ -114,6 +114,7 @@ def teardown
       end
 
       test 'register a new extension' do
+        add_to_config "config.assets.precompile = []"
         add_to_config %q{ config.annotations.register_extensions("scss", "sass") { |annotation| /\/\/\s*(#{annotation}):?\s*(.*)$/ } }
         app_file "app/assets/stylesheets/application.css.scss", "// TODO: note in scss"
         app_file "app/assets/stylesheets/application.css.sass", "// TODO: note in sass"