Automatic update of the *.json files.

[dalf]

The files generated by the script httpobs-regen-hsts-preload haven't been updated since February 2020.

If it is not a security issue (?), if it makes sense, a way to reduce the maintenance cost, is to run httpobs-regen-hsts-preload automatically using travis.

For example, the hstspreload project runs a cron.sh script that calls build.py to update the source code and then commit / push.

[EDIT] see dalf@1352da5