Update SASS and Bourbon

Updated Sass to 3.2.9 and Bourbon to 3.1.8. This required a few changes to our .scss files, because they were mixins that were deprecated or removed in the latest version of Bourbon. Running `rake assets` still produces lots of warnings, but the build appears to be successful: I need a designer or two to verify that the new branch still produces CSS that is comparable to the old branch.