fix yojson lower bound

Author: hhugo

packages/js_of_ocaml-compiler/js_of_ocaml-compiler.5.9.0/opam

@@ -23,7 +23,7 @@ depends: [
   "menhir"
   "menhirLib"
   "menhirSdk"
-  "yojson"
+  "yojson" {>= "1.6.0"}
   "odoc" {with-doc}
 ]
 depopts: ["ocamlfind"]