update pa_deriving_json

use the new deriving-ocsigen (0.5)
it allows to register new "predefs" so it's easier to work with external libs unaware of deriving.

```
let _ = Pa_deriving_Json.register_predefs ["Yojson";"Safe";"json"] [ "myyojson"]
```

It breaks compatibility with deriving-ocsigen.0.3*..