Use SCSS import vs sprockets require for compatibility with bourbon

Hey Redactor-Rails Maintainers,

I ran into an issue with my Rails 4 app (but could happen to 3.2 users as well). Instead of using sprockets `requires`, files in my application.css, as recommended by Thoughtbot's Bourbon project I use SCSS's `@import` feature and rename my applicaton.css to application.css.scss.

When I tried to import redactor-rails stylesheets, I got unreadable errors (I think because it uses sprockets inline which vanilla SCSS can't read), so I changed everything over to normal scss imports which work fine. I believe this should be backwards compatible with anyone sprockets so the README.md will not have to change.

Let me know what this needs to merge.