A working CDN config for galetahub/ckeditor which doesn't copy/precompile CKEditor's own assets.

assets.rb

# config/initializers/assets.rb

# ...

# The last two are for the file browser
Rails.application.config.assets.precompile += %w(ckeditor/config ckeditor/application ckeditor/filebrowser/*)

ckeditor.coffee.erb

# app/assets/javascripts/ckeditor.coffee.erb

# Another option is including these directly in your relevant views:
#   = javascript_include_tag Ckeditor.cdn_url
#   = javascript_include_tag 'ckeditor/config'
# I've chosen to implement the whole thing in JS. Going this route, 
# you might experience some FOUC. If that's an issue for you, usual FOUC
# workarounds apply, e.g. you can block the relevant textareas with a
# loading animation or something.

$ = jQuery

# https://api.jquery.com/jquery.getscript/#entry-examples
jQuery.cachedScript = (url, options) ->
# Allow user to set any option except for dataType, cache, and url
  options = $.extend(options or {},
    dataType: 'script'
    cache: true
    url: url)
  # Use $.ajax() since it is more flexible than $.getScript
  # Return the jqXHR object so we can chain callbacks
  jQuery.ajax options

$ ->
  $editors = $("textarea.html-editor")
  if $editors.length
    $.cachedScript("<%= Ckeditor.cdn_url %>").done ->
      $editors.each ->
        CKEDITOR.replace(this, {
          customConfig: "<%= javascript_path 'ckeditor/config' %>",
          # ...
        })

ckeditor.rb

# config/initializers/ckeditor.rb

Ckeditor.setup do |config|
  # ...

  config.cdn_url = '//cdn.ckeditor.com/4.5.6/standard/ckeditor.js'
  config.run_on_precompile = false
end