Use `Sass.load_paths` if `ENV['SASS_PATH']` not available.

Hey there,

unfortunately the commit https://github.com/thoughtbot/bourbon/commit/88fcaf30ff631dd9954462590f5aa3ea2ae00fce breaks the integration with https://github.com/locomotivecms/wagon.

Maybe @metaskills should look over the patch, whether it is ok to fall back to `Sass.load_paths` if `ENV['SASS_PATH']` not available.

PS: Many thanks for Bourbon!