Support 3.4-index() in exports() function

Updating the exports() function to work with Sass 3.2/3.3 and libsass 2.0 which return false for index() AND add support for Sass 3.4 and libsass 3.0 with the new behaviour.

Discussed in issue #5932 and sass/libsass#538
ah, thanks for the in-depth explanation :-)