Fix attach-refinery-{images|resources}-with-dragonfly initialization ord...

In our setup, `after: :load_config_initializers` causes the `attach-refinery-{images|resources}-with-dragonfly` initializers to be run after `:build_middleware_stack`, which effectively breaks image/resource loading. In master, there is a fix for this, which uses `before: :finisher_hook`, but this may still cause the initializer to be run after `:build_middleware_stack`. 

For details, cf. http://guides.rubyonrails.org/configuring.html#initializers