Decreased opacity for down arrow in disabled state

This was done as the result of user feedback (see issue JedWatson#1179). A user requested that we remove the arrow entirely but I felt that made the control look like an <input> instead of a <select>.

less/control.less

@@ -38,6 +38,7 @@
 	&.is-disabled .Select-arrow-zone {
 		cursor: default;
 		pointer-events: none;
+		opacity: 0.35;
 	}
 }