Latest plugin version strings in GCS may go out of sync briefly

This is a fairly minor issue, but I'm opening this for documentation purposes.

Running `update_gcs_latest.sh` as part of release may fail.
```
$ ./jib-maven-plugin/scripts/update_gcs_latest.sh 2.2.0
Copying file://jib-maven [Content-Type=application/octet-stream]...
/ [1 files][ 19.0 B/ 19.0 B]
Operation completed over 1 objects/19.0 B.
Updated ACL on gs://jib-versions/jib-maven
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 19 100 19 0 0 195 0 --:--:-- --:--:-- --:--:-- 193
Version update failed
```

Chances are that the file is actually updated but the old file cached is served for a while. It continued to serve the old version for a few minutes in my case.
```
$ gsutil cp gs://jib-versions/jib-maven file
Copying gs://jib-versions/jib-maven...
/ [1 files][ 19.0 B/ 19.0 B]
Operation completed over 1 objects/19.0 B.
$ cat file
{"latest":"2.2.0"}
$ curl https://storage.googleapis.com/jib-versions/jib-maven
{"latest":"2.1.0"}
```