Upgrade Opis or move to justinrainbow/json-schema

[dafeder]

We are stuck at opis/json-schema 1.x which is now quite old. It seems like it would be a good idea to move to the latest version... although, that version is now two years old too and appears to be abandoned or at least on hiatus. OTOH justinrainbow/json-schema is, despite maintainership issues, continuing to move forward thanks to community support. It is a dependency of composer, so there is motivation to keep it alive. And we use it elsewhere in DKAN, so it might be more consistent/easy to consolidate on one JSON validation library. The justinrainbow library is also simpler and easier to use IMO, especially for things like type coersion (which it might be useful to have RootedJsonData perform in some cases).

[dafeder]

There was an attempt to upgrade: #13