Update gcloud versions (#360)

Update latest gcloud versions

Author: google-github-actions-bot

data/versions.json

@@ -493,5 +493,6 @@
   "536.0.0",
   "536.0.1",
   "537.0.0",
-  "538.0.0"
+  "538.0.0",
+  "539.0.0"
 ]