Remove all uses of `selector &` to support nested import?

Created by: glebm

See https://github.com/sass/sass/issues/1817 for the context on this.

---

If we want to support namespaced import, i.e.:

.twbs {
  @import 'bootstrap';
}

We cannot use selector &, see https://github.com/sass/sass/issues/1817. This is one ugly workaround, but it works.

Requires libsass 3.3 (due to selector-unify and selector-nest) which has not been released yet (but soon hopefully), so travis will fail.