Fixing minimum phpunit version

Author: moufmouf

composer.json

@@ -20,7 +20,7 @@
         "graphp/algorithms": "~0.8.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "~4.0",
+        "phpunit/phpunit": "^4.8.36",
         "satooshi/php-coveralls": "^1.0"
     }
 }