Future friendly retina-mq

Using 'dpi' as a unit should be the way to go in the future

select2.css

@@ -556,7 +556,7 @@ disabled look for already selected choices in the results dropdown
 
 /* Retina-ize icons */
 
-@media only screen and (-webkit-min-device-pixel-ratio: 1.5) {
+@media only screen and (-webkit-min-device-pixel-ratio: 1.5), only screen and (min-resolution: 144dpi)  {
   .select2-search input, .select2-search-choice-close, .select2-container .select2-choice abbr, .select2-container .select2-choice div b {
       background-image: url('select2x2.png') !important;
       background-repeat: no-repeat !important;