allow phpunit 6 and 7

Author: dbu

composer.json

@@ -29,7 +29,7 @@
     "require": {
         "php": "^5.6|^7.0",
         "phpcr/phpcr-implementation": "2.1.*",
-        "phpunit/phpunit": "^5.7"
+        "phpunit/phpunit": "^6.4 || ^7.0"
     },
     "autoload": {
         "psr-4": {