Tell gulp-ruby-sass to disable sourcemaps

This is a temporary workaround for the following issue:
https://github.com/sindresorhus/gulp-autoprefixer/issues/20

Signed-off-by: Alex Coles <alex@alexbcoles.com>

frontend/gulpfile.js

@@ -69,6 +69,7 @@ gulp.task('fonts', function() {
 gulp.task('sass', function() {
   return gulp.src('../app/assets/stylesheets/default.css.sass')
     .pipe(sass({
+      'sourcemap=none': true,
       bundleExec: true,
       loadPath: [
         './bower_components/foundation-apps/scss',