Update required version of google/cloud-core (#1219)

Author: jdpedrie

composer.json

@@ -22,7 +22,7 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0",
-        "google/cloud-core": "^1.14",
+        "google/cloud-core": "^1.23",
         "phpdocumentor/reflection": "^3.0"
     },
     "suggest": {