Use absolute path to require the sass file

I was trying to figure out why my application is working and just found the reason.

In my application I'm overriding a bootstrap-sass file. As you can see [bootstrap-sass require all the files using absolute path](https://github.com/twbs/bootstrap-sass/blob/v2.3.2.2/vendor/assets/stylesheets/bootstrap/bootstrap.scss#L12). Turns out this is the reason for my application being working.

I think this behavior is because sass-rails when calling [the `depend_on`](https://github.com/rails/sass-rails/blob/master/lib/sass/rails/importer.rb#L77) is using the relative path only.

But I guess you already found this same problem.

I'll try to dig deeper on the sprockets/sass-rails gem.