Update deps

Updates dolmen to 0.9.0, restricts Core's version to < v0.16.0 because newer versions of Core require OCaml 4.14.0, while Colibri2 requires OCaml 4.12.0.

colibri2.opam

@@ -20,7 +20,7 @@ depends: [
   "dolmen_type" {>= "0.9"}
   "dolmen_loop" {>= "0.9"}
   "cmdliner" {>= "1.1.1"}
-  "base" {>= "v0.14.2"}
+  "base" {>= "v0.14.2" & < "v0.16.0"}
   "gen" {>= "1.0"}
   "re" {>= "1.10.3"}
   "ppx_here" {>= "v0.14.0"}