Added PHP version requirement to composer.json.

These PHP version should not be used anyway.

Author: frenck

composer.json

@@ -23,6 +23,7 @@
     "source": "https://github.com/DealerDirect/php-qa-tools"
   },
   "require": {
+    "php": ">=5.6,<8.0-dev",
     "phpmd/phpmd": "^2.2.0",
     "phpunit/phpunit": "^5.5.0"
   },