Remove experimental variable and mixin

Duplicating the work done in [this zurb/foundation commit](https://github.com/zurb/foundation/commit/3b3c67d4573ed69f5b8fc0d1ea54e07cf6060a8b). `@experimental` breaks all stylesheets that include both Foundation and Compass. This prevents users of the foundation-rails gem from updating to take advantage of new features or fix bugs that have been addressed.