updated for version 7.4.168

Problem:    Can't compile with Ruby 2.1.0.
Solution:   Add support for new GC. (Kohei Suzuki)

src/version.c

@@ -738,6 +738,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    168,
 /**/
     167,
 /**/