Update exports to support null index result to prevent break in rails 4.2

Updating to rails 4.2 breaks foundation because of a sass update. The cause of this break can be found at: https://github.com/sass/sass/commit/79bad1ff473cc06c66b85086403f4a1905938e37

The new implementation is based on: https://github.com/wilsonpage/sass-import-once/blob/master/_sass-import-once.scss