Fix for Dragonfly >=1

Several people have forked and removed the Dragonfly version restriction but no one has applied the proper fix to avoid loading dragonfly/rails/images so here it is. There isn't really a direct equivalent in >=1 as a little more configuration is required so let's just assume that Dragonfly has been setup correctly in recent versions.

I have also fixed the I18n translations as these didn't seem to be scoped properly.

This is my first time using this plugin and the forms look a little wonky to me but I haven't tried against older Active Admin versions so maybe it's supposed to look like that. Everything seems a little too far to the right because of the nested ol lists. At least it works. :)