ship phpunit 7 by default

composer.json

@@ -15,7 +15,7 @@
         "nunomaduro/collision": "~1.1",
         "fzaninotto/faker": "~1.4",
         "mockery/mockery": "~1.0",
-        "phpunit/phpunit": "~6.0",
+        "phpunit/phpunit": "~7.0",
         "symfony/thanks": "^1.0"
     },
     "autoload": {