Update select2.css

-ms-box-sizing does not exist, Konqueror does not need prefix.
-ms-text-overflow does not exist, Opera does not need prefix.
-moz-border-radius and -webkit-border-radius not needed anymore.
-moz-background-clip and -webkit-background-clip not needed anymore.
-moz-box-shadow not needed anymore (-webkit still used for Android 2.3)
Merge pull request #1477 from Ugoku/patch-1

Update select2.css