PHPUnit 6 instead of 4, to get rid of wrapper

The TestCase wrapper caused PhpStorm to show warnings about
multiple implementations

Fixes #188

Improved some type hints in DocBlocks

tests/PHPUnit_Framework_TestCase.inc.php

@@ -1,11 +0,0 @@
-<?php
-
-namespace PHPUnit\Framework;
-
-// Only needed to keep unit test code compatible with older PHPUnit versions
-
-/** @noinspection PhpUndefinedClassInspection */
-/** @noinspection AutoloadingIssuesInspection */
-class TestCase extends \PHPUnit_Framework_TestCase {
-
-}