Use Countdown from multicore-bench

Author: polytypic

bench/countdown.ml

@@ -205,7 +205,7 @@
     :with-test))
   (multicore-bench
    (and
-    (>= 0.1.5)
+    (>= 0.1.7)
     :with-test))
   (multicore-magic
    (and

bench/countdown.mli

@@ -30,7 +30,7 @@ depends: [
   "domain_shims" {>= "0.1.0" & with-test}
   "js_of_ocaml" {>= "5.4.0" & with-test}
   "mdx" {>= "2.4.0" & with-test}
-  "multicore-bench" {>= "0.1.5" & with-test}
+  "multicore-bench" {>= "0.1.7" & with-test}
   "multicore-magic" {>= "2.3.0" & with-test}
   "multicore-magic-dscheck" {>= "2.3.0" & with-test}
   "ocaml-version" {>= "3.6.4" & with-test}