Fixed layout path for Ckeditor::ApplicationController for enabling rails 4 file uploading

```Ckeditor::ApplicationController``` class has no layout path definition,

so in Rails 4, ```Ckeditor::PicturesController < Ckeditor::ApplicationController``` uses your rails app's ```application.html.erb``` instead of ```ckeditor/application.html.erb``` as it's layout so required assets for uploading files are not loaded.

so I added ```layout 'ckeditor/application'``` in ```Ckeditor::ApplicationController```

combined with pull request #276, file uploading in rails 4 will work well.