Use dppx instead of non-existing ppx resolution unit

Author: mkurz

select2.css

@@ -628,7 +628,7 @@ html[dir="rtl"] .select2-search-choice-close {
 
 /* Retina-ize icons */
 
-@media only screen and (-webkit-min-device-pixel-ratio: 1.5), only screen and (min-resolution: 2ppx)  {
+@media only screen and (-webkit-min-device-pixel-ratio: 1.5), only screen and (min-resolution: 2dppx)  {
     .select2-search input,
     .select2-search-choice-close,
     .select2-container .select2-choice abbr,