Fix for BC break introduced in Sass 3.3.0rc3

The latest Sass master introduces a BC break with the return value of `index` breaking `export`.

The break in question was introduced here https://github.com/nex3/sass/commit/79bad1ff473cc06c66b85086403f4a1905938e37

This patch fixes the issues maintaining compatibility with 3.3.0rc2 and below.  

This fixes #7.