[csharp] Remove Unity 4 and fix Unity 5+ check.

spine-csharp/src/SkeletonJson.cs

@@ -28,7 +28,7 @@
  * POSSIBILITY OF SUCH DAMAGE.
  *****************************************************************************/
 
-#if (UNITY_5 || UNITY_4_0 || UNITY_4_1 || UNITY_4_2 || UNITY_4_3 || UNITY_4_4 || UNITY_4_5 || UNITY_4_6 || UNITY_4_7 || UNITY_WSA || UNITY_WP8 || UNITY_WP8_1)
+#if (UNITY_5 || UNITY_5_3_OR_NEWER || UNITY_WSA || UNITY_WP8 || UNITY_WP8_1)
 #define IS_UNITY
 #endif