Change OCaml constraint to 4.03.0

Author: lpw25

opam

@@ -10,7 +10,7 @@ dev-repo: "http://github.com/ocaml-doc/octavius.git"
 bug-reports: "https://github.com/ocaml-doc/octavius/issues"
 tags: ["doc" "ocamldoc" "org:ocaml-doc"]
 
-available: [ ocaml-version >= "4.01.0"]
+available: [ ocaml-version >= "4.03.0"]
 depends: [
   "ocamlfind" {build}
   "ocamlbuild" {build}