update json key

[lordfolken]

@rawtaz
I think we should design this how its supposed to work.

The current approach basically is:
update: - date the file is valid from.

This (although currently broken) should add an update field to the repository and then xcsoar's repository should offert this as an update.

I see an issue with this approach:

if the date is in the future:
A client with an existing file can then wait until the day and then update it.
A new client, can only download the new file, but not the old one which may be still the valid file for the current day.

If the date is in the past:
A client with an existing file will show it as update able
A client without the file is no problem.

A second problem is that this update tag has to be maintained here in this repo. An approach at automation would be to check the modifled date and add that to the repository db.

curl -sI http://foo.com/bar.zip | grep 'Last-Modified' | cut -d' ' -f 2-

Potentially we would have to add a second field "validfrom" to be more precise.