Update PhpUnit version in composer.json

[Ognj3n]

The latest stable version of PhpUnit is 7.*.

However installation source code requires 5.7 PhpUnit version, which can make debugging of PhpUnit tests pretty messy for first timers in October CMS.

Will be glad to see this updated.

[LukeTowers]

@Ognj3n feel free to make a PR to octobercms/october and octobercms/library and any RainLab plugins using unit testing to upgrade the version.

[viamage]

Latest PhpUnit is 7.x, but php-selenium requires php-unit <7, so 6.5 I guess.

I wonder if we should wait for giorgiosironi/phpunit-selenium#422 or upgrade to 6.x for now? I guess we can proceed with 6.x to resolve #3360 at least.

EDIT:
Ah actually #3051 pulling this would resolve it and make us able to go for phpunit7

[github-actions]

This issue will be closed and archived in 3 days, as there has been no activity in the last 30 days. If this issue is still relevant or you would like to see action on it, please respond and we will get the ball rolling.

[w20k]

Was upgraded by @bennothommo to 6.5 version (https://github.com/octobercms/october/blob/master/composer.json#L46)
The next major upgrade will be, most probably done, after migration to a Laravel 6.0.

[bennothommo]

Upgraded to PHPUnit 8 as part of #4893.