Detect that the last update was too long ago

[cristan]

https://storage.googleapis.com/ov-fiets-updates/locations.json could be outdated because the script stopped working. In fact, I'm pretty sure it stopped right now.

The app could detect the last update was from too long ago and don't show the amount in the overview. The detail screen should be fine: that's coming from the NS and that works completely differently.