Allow higher versions of PHPUnit

Author: rbibby

composer.json

@@ -23,7 +23,7 @@
         }
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.7",
+        "phpunit/phpunit": ">=5.7",
         "squizlabs/php_codesniffer": "3.*",
         "fzaninotto/faker": "^1.4"
     }