After I briefly reviewed the changes made in v5.0, it appears that in README the section regarding CDN and overriding config.js is no longer relevant, as the gem only support loading the editor as well as the config.js via CDN now. Therefore, it's pretty misleading.
Figured that I misunderstood the section in question. Thought that the entire CDN section is optional for setup. Turned out that the include_javascript_tag is required.
After I briefly reviewed the changes made in v5.0, it appears that in README the section regarding CDN and overriding config.js is no longer relevant, as the gem only support loading the editor as well as the config.js via CDN now. Therefore, it's pretty misleading.