Updated sass-import-once to the latest version

The [sass-import-once](https://github.com/wilsonpage/sass-import-once/blob/master/_sass-import-once.scss) code included in `functions.scss` has been updated a month ago, it would be nice to update it in Foundation.

For a reason I don't understand, the old version currently in use does not include any of the blocks prepended with `@include exports(...)` in my CSS output (using `scss.bat` on Windows), whereas the new version works as expected.

The [relevant commit](https://github.com/wilsonpage/sass-import-once/commit/a86c6f364a5a3a577c5357ebd21fa7a7aa583437) suggests it's about supporting `null` values and not just `false`.