Support both PHPunit 6.x and 7.x

Author: curry684

composer.json

@@ -32,7 +32,7 @@
         "doctrine/orm": "^2.5",
         "friendsofphp/php-cs-fixer": "^2.7",
         "mongodb/mongodb": "^1.2",
-        "phpunit/phpunit": "^7.1",
+        "phpunit/phpunit": "^6.5|^7.1",
         "ruflin/elastica": "^6.0",
         "symfony/browser-kit": "^3.0|^4.0",
         "symfony/css-selector": "^3.0|^4.0",