Change ckeditor path to relative

Ckeditor path needs to be relative to allow image_tag helper generate the right path

`` <%= image_tag '/ckeditor/filebrowser/images/thumbs/pdf.gif’>``
generates
``<img src="/ckeditor/filebrowser/images/thumbs/pdf.gif">``

``<%= image_tag 'ckeditor/filebrowser/images/thumbs/pdf.gif’>``
generates
``<img src="/assets/ckeditor/filebrowser/images/thumbs/pdf.gif">``