update minimal php requirement to the 7.1

composer.json

@@ -11,7 +11,7 @@
         }
     ],
     "require": {
-        "php": "^7.0",
+        "php": "^7.1",
         "symfony/serializer": "^3.0|^4.0",
         "paragonie/random_compat": "*"
     },