Update to dragonfly ~>1.0

This is a patch to support the current version of [dragonfly 1.0.7](https://github.com/markevans/dragonfly). So that the current version of [dragonfly-s3_data_store](https://github.com/markevans/dragonfly-s3_data_store) would work. I have also submitted a [pull request](https://github.com/stefanoverna/activeadmin-dragonfly/pull/10) for [activeadmin-dragonfly](https://github.com/stefanoverna/activeadmin-dragonfly)