Cache data written! You will no longer need to manually authorize this
Dropbox account via an authorization URL on this machine.

Note: If you run Backup with this Dropbox account in another location,
or on other machines, you will need to either authorize them the same
way, or simply copy over the cache file from:
<%= @cached_file %>
to the cache directory on your other machines to use this Dropbox
account there as well.