Upgraded to phpunit/phpunit 7.x to allow sebastian/diff 3.x compatibility

Author: Ocramius

composer.json

@@ -21,7 +21,7 @@
         "ext-reflection": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.0",
+        "phpunit/phpunit": "^7.0",
         "phpbench/phpbench": "^0.16.1"
     },
     "autoload": {