[opam] Use version to provide better package bounds.

I copied this from Ralf Jung's submission to the Coq Package index,
let's see if it works.

1 files changed, 1 insertions, 1 deletions

diff --git a/coq.opam b/coq.opam
index 02c57b8683..da3f1b518d 100644
--- a/coq.opam
+++ b/coq.opam
@@ -20,7 +20,7 @@ license: "LGPL-2.1"
 
 depends: [
   "ocaml"     {         >= "4.05.0" }
-  "dune"      { build & >= "1.4.0"  }
+  "dune"      { build & >= "1.6.0"  }
   "ocamlfind" { build }
   "num"
 ]