Bump grunt-sass to ^1.0.0

/cc @cvrebert @patrickhlauke

Because we use `position: absolute` on our inputs, when there’s no
label the `.checkbox` ends up having no computed height. To avoid
rendering errors when there’s no label text, we reset the `position` to
`static` for normal rendering.

We have it set to `1` right now just to avoid compilation errors, but
it’s been replaced everywhere with our new spacer classes anywho. We’ll
likely want to remap that var to custom component vars though (e.g.,
`$pagination-margin` instead of `$spacer-y`).

Wrap `@viewport` with `@at-root` for nested import support

$enable-flex

See https://github.com/twbs/bootstrap-sass/pull/805