Added .scss code for "checkbox not showing on chrome" (issue 28)

Following the issue at https://github.com/littlesparkvt/flatstrap/issues/28 I noticed that the commit that resolved it was only for the .less code (ab7054b1ca1475f3e03a981459a33fe971008479)

Since I'm using your repository for my gem https://github.com/lucatironi/flatstrap-sass (based on https://github.com/thomas-mcdonald/bootstrap-sass), I thought to change also the .scss code.

Thanks for your consideration and your amazing work.