update autoprefixer browsers

Author: Helen

tasks/config/autoprefixer.js

@@ -1,7 +1,7 @@
 module.exports = function(grunt) {
     return {
         options: {
-            browsers: ['last 4 versions', 'ie 8', 'ie 9', 'Android 2.3']
+            browsers: ['last 4 versions', 'ie 8', 'ie 9', 'iOS >= 6.0', 'Android 2.3', 'Android 4', 'last 2 Chrome versions']
         },
         multiple_files: {
             flatten: true,