Update php version constraint

Author: joelwurtz

composer.json

@@ -15,7 +15,7 @@
         }
     ],
     "require": {
-        "php": ">=7.1",
+        "php": "^7.1",
         "phpunit/phpunit": "^7.0",
         "php-http/message": "^1.0",
         "guzzlehttp/psr7": "^1.0",