Restrict to OCaml >= 4.04

Author: SimonJF

packages/links/links.0.6/opam

@@ -5,6 +5,7 @@ homepage: "http://www.links-lang.org"
 bug-reports: "https://github.com/links-lang/links/issues"
 license: "GPL-2"
 dev-repo: "https://github.com/links-lang/links.git"
+available: [ ocaml-version >= "4.04" ]
 build: [make "nc"]
 install: [
   ["cp" "links" "%{links:bin}%"]