Can now @import from subfolder

Bootstrap itself did this same change to fix load path issues with SASS
compiler via PR: https://github.com/thomas-mcdonald/bootstrap-sass/pull/258/files

[ActiveAdmin](https://github.com/reverbhq/active_admin/tree/switch-to-bootstrap) is migrating to this and we were unable to include this CSS into a print stylesheet in a subfolder.

`app/stylesheets/active_admin/print.css.scss`

```sass
@import "formtastic-plus-bootstrap";
```

This would fail b/c when pulling this in all of the mixins inside the `formtastic-plus-bootstrap` gem could not be located.  Importing the main file from a SASS file in the root of `app/stylesheets` worked fine.