Upgrade to PHPUnit 6.x (composer require phpunit/phpunit '^6.0' --update-with-dependencies)

Author: petrkotek

composer.json

@@ -25,7 +25,7 @@
     "require": {
         "php": ">=5.6",
         "bigcommerce/injector": "@stable",
-        "phpunit/phpunit": "^5.6"
+        "phpunit/phpunit": "^6.0"
     },
     "autoload": {
         "psr-4": {