Modernize code examples in README.md

[derrabus]

This PR modernizes the code examples and the documentation on mock objects.

• Current PHPUnit releases use the namespaced interface PHPUnit\Framework\MockObject\MockObject for mocks. The non-namespaced PHPUnit_Framework_MockObject_MockObject is legacy and has been removed a long time ago.
• Current PHPUnit releases require the setUp() method in tests to have a void return type.
• PHP nowadays supports native intersection types which should be what we recommend to use in the README file. PHPDoc should only be a fallback if the projects needs to support legacy PHP runtimes.

[ondrejmirtes]

Awesome, thank you!