Add lower bound on re and fix dune subst

Author: shonfeder

packages/obelisk/obelisk.0.8.0/opam

@@ -13,10 +13,10 @@ depends: [
   "ocaml" {>= "4.08"}
   "dune" {>= "2.2.0"}
   "menhir" {>= "20190613"}
-  "re"
+  "re" {>= "1.7.2"}
 ]
 build: [
-  ["dune" "subst"] {pinned}
+  ["dune" "subst"] {dev}
   [
     "dune"
     "build"