bumps minimum guzzle version to 5.3.1 for php7.1 compatibility (#1241)

Author: bshaffer

composer.json

@@ -12,7 +12,7 @@
         "firebase/php-jwt": "~2.0|~3.0|~4.0",
         "monolog/monolog": "^1.17",
         "phpseclib/phpseclib": "~0.3.10|~2.0",
-        "guzzlehttp/guzzle": "~5.2|~6.0",
+        "guzzlehttp/guzzle": "~5.3.1|~6.0",
         "guzzlehttp/psr7": "^1.2"
     },
     "require-dev": {