add support for PHPUnit 7

Author: xalopp

composer.json

@@ -22,7 +22,7 @@
         "source": "https://github.com/mayflower/mo4-coding-standard"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.0 || ^5.0 || ^6.0"
+        "phpunit/phpunit": "^4.0 || ^5.0 || ^6.0 || ^7.0"
     },
     "require": {
         "squizlabs/php_codesniffer": "^3.2.3",