patch 8.1.0421: MS-Windows: Ruby path is wrong for Ruby 1.9 and later

Problem:    MS-Windows: Ruby path is wrong for Ruby 1.9 and later.
Solution:   Let -I argument depend on Ruby version. (Ken Takata, closes #3461)

src/version.c

@@ -794,6 +794,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    421,
 /**/
     420,
 /**/