Fixes issue with ckeditor BASEPATH in subdir (passenger) enviroment

Hi. After all this time using github it is my first try of contribution so feel free to criticize ;)

This little fix add an `ENV["RAILS_RELATIVE_URL_ROOT"]` if exist to `window['CKEDITOR_BASEPATH']` in an override.js file (for rails 4). 

Hardcoded path to `/assets/ckeditor/` unabled to found assets of ckeditor if app is runing in a subdir.   Now it gather an rails ENV variable (delivered by passenger in `PassengerBaseURI` option)