Turn off clean-css' advanced optimizations.

Works around #13914.

We might give it another go in the future but the stuff this breaks over the time does not justify having it enabled. The gain in size is negligible:

dist/css/bootstrap.min.css: 108.65 kB → 109.5 kB
dist/css/bootstrap-theme.min.css: 18.84 kB → 18.86 kB
docs/assets/css/docs.min.css: 20.44 kB → 20.61 kB

Gruntfile.js

@@ -232,7 +232,8 @@ module.exports = function (grunt) {
     cssmin: {
       options: {
         compatibility: 'ie8',
-        keepSpecialComments: '*'
+        keepSpecialComments: '*',
+        noAdvanced: true
       },
       core: {
         files: {