to_json / of_yojson attribute example doesn't compile

The compilation of the example [here](https://github.com/ocaml-ppx/ppx_deriving_yojson#to_yojson--of_yojson) fails with
```
File "_none_", line 1:
Error: Unbound value StringMap.to_yojson
```

My workaround right now is to define:
```
type attrs = string StringMap.t
let attrs_to_json m = StringMap.bindings m |> [%to_yojson: (string * string) list]
```