restricted phpunit6

Author: lesstif

composer.json

@@ -10,7 +10,7 @@
         "vlucas/phpdotenv": "~2.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "~4.4",
+        "phpunit/phpunit": ">=4.4 <6",
         "mockery/mockery": "^0.9.4",
         "symfony/var-dumper": "~2.8|~3.0"
     },