Downgrade natcmp version. Now it builds with GDC 5.3.0

Author: azbukagh

dub.json

@@ -10,6 +10,6 @@
 		"./test"
 	],
 	"dependencies": {
-		"natcmp": "~>1.3.1"
+		"natcmp": "1.2.3"
 	}
 }