Use `SASS_PATH` vs `Sass.load_paths` to avoids loading race conditions.

I am writing an internal style gem that uses Bourbon, Susy, and Team-Sass/breakpoint. Each have their own ruby gems and each follow current Sass 3.3 [best practice](https://github.com/ericam/susy/blob/master/lib/susy.rb) for adding their path for Sass to see using `SASS_PATH`, except for Bourbon. From Sass's [changelog](https://github.com/nex3/sass/blob/stable/doc-src/SASS_CHANGELOG.md)

> The automatic placement of the current working directory onto the Sass load path is now deprecated as this causes unpredictable build processes. If you need the current working directory to be available, set SASS_PATH=. in your shell's environment.

Not doing this and calling `Sass.load_paths` will nullify any gem doing the right thing with `SASS_PATH` that is unlucky enough to be loaded after Bourbon since calling `Sass.load_paths` will only parse the `SASS_PATH` just once, [see here](https://github.com/nex3/sass/blob/stable/lib/sass.rb#L42-L48). This patch fixes that by doing the write thing and using `SASS_PATH`.