change dep to latest google php pubsub only, not entire cloud lib

Author: Bertus Steenberg

composer.json

@@ -11,7 +11,7 @@
   "require": {
     "php": ">=5.6.0",
     "superbalist/php-pubsub": "^2.0",
-    "google/cloud": "^0.95.0"
+    "google/cloud-pubsub": "^1.21.0"
   },
   "autoload": {
     "psr-4": {