Use single quotes for retina image url

This is how all the other images are being referenced.

select2.css

@@ -519,7 +519,7 @@ disabled look for already selected choices in the results dropdown
 
 @media only screen and (-webkit-min-device-pixel-ratio: 1.5) {
 	.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-image: url('select2x2.png') !important;
 		background-repeat: no-repeat !important;
 		background-size: 60px 40px !important;
 	}