Force dune version for now
Force dune version for now

With dune 1.7.1 in ocaml 4.06.1 switch, `opam install eliom=6.3.0`
fails. It boils down to js_of_ocaml-lwt not being installed
correctly. Indeed, the META file generated doesn't include the logger
sublibrary. It seems the logger sublibrary is not built at all. With
dune 1.6.2, the sublibrary is properly built, and its META stanza is
properly generated.

See https://github.com/ocaml/dune/issues/1847

glondu/belenios,glondu/belenios,glondu/belenios,glondu/belenios,glondu/belenios