Update composer.json

Author: GrahamCampbell

composer.json

@@ -28,7 +28,7 @@
         "bamarni/composer-bin-plugin": "^1.8.2",
         "guzzlehttp/guzzle": "^7.5.1",
         "http-interop/http-factory-guzzle": "^1.2",
-        "php-http/mock-client": "^1.5"
+        "php-http/mock-client": "^1.5.1"
     },
     "autoload": {
         "psr-4": {