replace dev-requirement nyholm/psr7 to guzzlehttp/psr7 for php 7.0 support

Author: ddrv

composer.json

@@ -17,7 +17,7 @@
     "psr/log": "^1.0"
   },
   "require-dev": {
-    "nyholm/psr7": "^1.3",
+    "guzzlehttp/psr7": "^1.7",
     "phpunit/phpunit": "^6.5",
     "squizlabs/php_codesniffer": "^3.5",
     "webclient/fake-http-client": "^1.0"