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)
diff --git a/src/version.c b/src/version.c
index 992ed7f..c541dc9 100644
--- a/src/version.c
+++ b/src/version.c
@@ -795,6 +795,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    421,
+/**/
     420,
 /**/
     419,