Update rake task to create non digest assets

The original rake task did not work for me. The output was:
```
"Copy /tmp/build_0d494270-5e35-4650-85b6-f25d45c5e2b0/vendor/bundle/ruby/2.0.0/bundler/gems/ckeditor-6ac7f1eb21cb/vendor/assets/javascripts/ckeditor/contents.css to /tmp/build_0d494270-5e35-4650-85b6-f25d45c5e2b0/public/assets/ckeditor/contents.css"
```

Now it seems to work: 
```
cp public/assets/ckeditor/skins/moono/icons_hidpi-37c74fe2d448f1f89bae6e815940580e.png public/assets/ckeditor/skins/moono/icons_hidpi.png
```

The code is based on https://github.com/bastiaanterhorst/rich/blob/master/lib/tasks/rich_tasks.rake