Revert "Change version to v3.0.0"

This reverts commit a6a5dbd6.

include/rtdef.h

 /*
  * File      : rtdef.h
  * This file is part of RT-Thread RTOS
- * COPYRIGHT (C) 2006 - 2017, RT-Thread Development Team
+ * COPYRIGHT (C) 2006 - 2015, RT-Thread Development Team
  *
  *  This program is free software; you can redistribute it and/or modify
  *  it under the terms of the GNU General Public License as published by
@@ -49,8 +49,8 @@ extern "C" {
 /*@{*/
 
 /* RT-Thread version information */
-#define RT_VERSION                      3L              /**< major version number */
-#define RT_SUBVERSION                   0L              /**< minor version number */
+#define RT_VERSION                      2L              /**< major version number */
+#define RT_SUBVERSION                   1L              /**< minor version number */
 #define RT_REVISION                     0L              /**< revise version number */
 
 /* RT-Thread version */