Doesnt build

[idkjs]

This excellent demo project doesnt seem to build. I think it has to to do with opium being set to ```json
"opium" {>= "0.17.1"}

when you might want
```json
  "opium" {=>"0.18.0"}

The Opium.Std lib was removed in the last update, I think.

At any rate: when you run make run you get his error:

ke run
DATABASE_URL="postgresql://admin:secret@localhost:5432/ocaml_webapp" dune exec ocaml_webapp
File "server/lib/handlers.ml", line 139, characters 2-11:
139 |   not_found (fun _req -> respond' @@ basic_page [ Shared.PageNotFound.make () ])

I would love to see this running, if you have any feedback.

Thank for sharing.