Bump magic_str to that of Ocaml 4.12

https://github.com/ocaml/ocaml/blob/trunk/utils/HACKING.adoc
https://github.com/ocaml/ocaml/blob/4.12/utils/config.mlp
--- a/src/index.ml
+++ b/src/index.ml
@@ -15,7 +15,7 @@ type section = Code | Dlpt | Dlls | Prim
 
 type t = (section * int * int) list
 
-let magic_str = "Caml1999X023"
+let magic_str = "Caml1999X029"
 
 let parse ic =
   let magic_size = String.length magic_str in