Update PHPUnit

[mjaschen]

Currently an outdated version of PHPUnit (4.8) is referenced in composer.json.

It should be updated to a current release.

It's required to increase the minimum required PHP version as well (see #35), as PHPUnit doesn't support ancient PHP releases any longer.

[iamstuartwilson]

When I find some time (potentially over Christmas), I'd love to get test coverage up and some more examples in place for this with a docker container for running them etc to help people get up and running a bit quicker on this. Perhaps we can make a checklist of v2 features and the prioritise them to ensure it's not too wild or time consuming a thing to achieve

[iamstuartwilson]

...obviously this will require writing PHP for the first time in few years though 😅