Make mdx incompatible with ocaml-migrate-parsetree 2.0.0

Author: gpetiot

mdx.opam

@@ -24,7 +24,7 @@ depends: [
   "cmdliner" {>= "1.0.0"}
   "re" {>= "1.7.2"}
   "result"
-  "ocaml-migrate-parsetree" {>= "1.0.6"}
+  "ocaml-migrate-parsetree" {>= "1.0.6" & < "2.0.0"}
   "ocaml-version" {>= "2.3.0"}
   "odoc"
   "lwt" {with-test}