Update `opam` file for release.

opam

@@ -23,6 +23,6 @@ depends: [
 ]
 synopsis: "LR(1) parser generator"
 url {
-  src: "http://gallium.inria.fr/~fpottier/menhir/menhir-20181005.tar.gz"
-  checksum: "md5=14f46d918cf8571dd735cc22c4fc4811"
+  src: "http://gallium.inria.fr/~fpottier/menhir/menhir-20181006.tar.gz"
+  checksum: "md5=d2174b0d4bd8feb66a7484be6f1eed14"
 }