Use ">=" for the "php" requirement

Author: nicolas-grekas

composer.json

@@ -16,7 +16,7 @@
         }
     ],
     "require": {
-        "php": "^7.1.3",
+        "php": ">=7.1.3",
         "ext-xml": "*",
         "symfony/http-kernel": "^3.4|^4.0|^5.0",
         "symfony/twig-bridge": "^3.4|^4.0|^5.0",