Allow PHPUnit 7 series

Author: weierophinney

composer.json

@@ -35,7 +35,7 @@
         "php": "^5.6 || ^7.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.0.7 || ^5.7.14",
+        "phpunit/phpunit": "^5.7.27 || ^6.5.8 || ^7.1.2",
         "athletic/athletic": "^0.1",
         "zendframework/zend-stdlib": "^2.7.3 || ^3.0",
         "container-interop/container-interop": "^1.1.0",