Merge pull request #2 from CraigFe/remove-dune-build-directive

opam: remove the 'build' directive on dune dependency

mirage-kv-mem.opam

@@ -22,7 +22,7 @@ build: [
 
 depends: [
   "ocaml" {>= "4.05.0"}
-  "dune" {build}
+  "dune"
   "alcotest" {with-test}
   "ppx_deriving" {with-test}
   "mirage-clock" {>= "2.0.0"}