Fix wysiwyg.css linking

There is an error in assets wysiwyg.css linking in production environment when using assets precompilation:

Currently this file is linked directly in js (https://github.com/stefanoverna/activeadmin-wysihtml5/blob/master/app/assets/javascripts/activeadmin-wysihtml5/base.js.coffee#L44). That's why it's Not Found after precompilation. File simply does not exists in a public/ directory, but precompilled wysiwyg-some-long-hash.css exists!

Maybe there are some better solutions to fix this issue, but I've decided just to add .erb extension to base.js.coffee file to have an ability to use there Rails's  assets helpers.