Sass bourbon

Since WooCommerce is using Ruby Version of Sass via `grunt-contrib-sass` we are not forced to use [Bourbon 3.2.x](https://github.com/thoughtbot/bourbon#requirements) which node-bourbon is completely based. [sass-bourbon](https://github.com/axisthemes/sass-bourbon) is based upon the latest updates via bourbon :)

This Fixes #6709 :)