Update JSON config version to 3

Increased due to added QuickPlay section and slightly changed
loading code. Previous versions assumed certain JSON nodes were
present; now node existence is checked to prevent crashes.

src/cdogs/config_json.c

@@ -37,7 +37,7 @@
 #include "json_utils.h"
 #include "keyboard.h"
 
-#define VERSION "2"
+#define VERSION "3"
 
 
 static void LoadGameConfigNode(GameConfig *config, json_t *node)