This logic supports the following cases:
- Download a fresh apps.json and load it if missing
- Check if a local apps.json is old and get a fresh one if so
- Always save a local copy after downloading a fresh apps.json
The http.Head() call is faster than a http.Get() call (only carries back
respones headers) and aims to make the more general case more
performant: you have the latest copy of the apps.json and don't need to
download another one. This a direct port of our Bash implementation
logic.
Closes https://git.autonomic.zone/coop-cloud/go-abra/issues/9.
