Update `dragonfly` to the latest version and update corresponding template.

Hey there,

just a small update for `dragonfly`. The `protect_from_dos_attacks` option was replaced by `verify_urls ` which is enabled by default, see: https://github.com/markevans/dragonfly/commit/e88afeceb036fe4d44f7c7787c7e988e1350c2dc

I think the `require 'dragonfly' ` is not needed.