Upgrade for Dragonfly 1.0

Dragonfly::App[:refinery_resources] is deprecated - use Dragonfly.app (for the default app) or Dragonfly.app(:refinery_resources) (for extra named apps) instead.
Also, `content_disposition=` is no longer available in Dragonfly 1.0, we should use `response_header=` instead, see: https://github.com/markevans/dragonfly/wiki/Upgrading-from-0.9-to-1.0#misc-config-options