Remove PHPUnit dependency from `composer.json`

[aik099]

Right now there a dependency on PHPUnit in composer.json which:

• from one side is correct (library creates a test case class for PHPUnit)
• from other side is not (it's forcing local PHPUnit copy in project even if PHPUnit is installed globally on machine and used from there)

@stof , can we just remove PHPUnit dependency?

[stof]

I would say yes (but documenting it). This would let everyone choose how they want to install PHPUnit (and they won't use this library if they don't use PHPUnit anyway).
This is the approach I took in https://github.com/phpspec/prophecy-phpunit btw