fix uri.1.6.0 syntax

opam-admin check was failing:
Fatal error: exception OpamSystem.Internal_error("File /home/edwin/something/opam-repository/packages/uri/uri.1.6.0/opam: Bad format! Expected a formula list of the form [ "item" {condition}... ], got option(ident(ounit),{symbol(<) string("1.0.2")})")