Adapt fallback.json to the new format.

The process to deploy this should be:
  - Pause proxy creation in the cloudmaster (e.g. by renaming /home/lantern/cloudmaster.py),
  - Deploy the new client version,
  - Rebuild installers.
Upgrading Lantern in the proxies themselves may be a good idea, but it's not strictly necessary.  The controller handles old clients alright.