update the version number.

include/rtdef.h

@@ -27,6 +27,7 @@
  * 2018-12-28     armink       change version number to v3.1.2
  * 2019-03-14     armink       change version number to v3.1.3
  * 2019-06-12     armink       change version number to v3.1.4
+ * 2020-05-14     armink       change version number to v3.1.5
  */
 
 #ifndef __RT_DEF_H__

src/Kconfig

@@ -314,7 +314,7 @@ endmenu
 
 config RT_VER_NUM
     hex
-    default 0x30104
+    default 0x30105
     help
         RT-Thread version number