Update gcloud versions (#317)

Update latest gcloud versions

Author: google-github-actions-bot

data/versions.json

@@ -466,5 +466,6 @@
   "511.0.0",
   "512.0.0",
   "513.0.0",
-  "514.0.0"
+  "514.0.0",
+  "515.0.0"
 ]