[opam] fix Yaml minimal bound

opam

@@ -123,6 +123,7 @@ depends: [
   "ocamlfind" # needed beyond build stage, used by -load-module
   "ocamlgraph" { >= "1.8.8" }
   "why3" { >= "1.6.0" }
+  "yaml" { >= "3.0.0" }
   "yojson" {>= "1.6.0" & (>= "2.0.1" | !with-test)}
   "zarith" { >= "1.5" }