Merge pull request #1 from CraigFe/remove-dune-build-directive

opam: remove the 'build' directive on dune dependency

mirage-kv-unix.opam

@@ -14,7 +14,7 @@ build: [
 ]
 
 depends: [
-  "dune" {build & >= "1.0"}
+  "dune" {>= "1.0"}
   "ocaml" {>= "4.05.0"}
   "cstruct" {>= "3.2.0"}
   "mirage-kv-lwt" {>= "1.0.0"}