Update phpunit version

Author: bruli

composer.json

@@ -21,11 +21,10 @@
         }
     },
     "require": {
-        "php": ">=5.6",
+        "php": "^7.0",
         "friendsofphp/php-cs-fixer": "^2.1",
-        "phpunit/phpunit-mock-objects": "~3.0",
-        "phpunit/phpunit": "~5.0",
-        "phpunit/php-code-coverage": "~4.0",
+        "phpunit/phpunit-mock-objects": "~4.0",
+        "phpunit/phpunit": "~6.0",
         "mybuilder/phpunit-accelerator": "^1.1",
         "squizlabs/php_codesniffer": "~2.8",
         "phpmd/phpmd": "~2.6",