Support justinrainbow/json-schema ^6.0

[yariksheptykin]

I just tried to install this package and got v1.0.0 installed by composer. It looks like composer hat to fallback to this old version because we require"justinrainbow/json-schema": "^6.4.2", in our application. v1.0.0 is, however, not compatible with phpunit v9.x, which we are requiring for --dev. Is there any any issue with adding support for justinrainbow/json-schema v6? Maybe as "justinrainbow/json-schema": "^5.0 || ^6.0"?

[sidz]

I've raised a PR #61 (comment) so let see, maybe @martin-helmich will have a chance to take a look as soon as he can.