[7.0] Add ext-json dependency

[driesvints]

Was missing apparently.

[taylorotwell]

Is it even possible to have PHP 7.1 without the JSON extension? Isn't it built in?

[driesvints]

@taylorotwell yes but it's always a good idea to add them as a requirement to your composer.json to know exactly which libraries/extensions your project depends on. In the situations where you have a custom build of PHP this can be a good gotcha for your to know why you can't install a library like Passport. PHPStorm has started throwing warnings if these dependencies aren't added:

https://medium.com/@luis.barros.nobrega/ext-json-is-missing-in-composer-json-what-is-this-700a1fcad966