Add dragonfly ruby gem advisory

Please double check. I choosed version 1.0.7 as fixed cause of the latest updates -> https://github.com/markevans/dragonfly/blob/master/History.md.

BUT iam not sure if we can call 1.0.7 a fixed version cause of this post http://seclists.org/fulldisclosure/2014/Aug/68:

"...many websites out there will still disable the verification... Note that this vulnerability is still exploitable if the attacker is unable to upload images, by using the generators to "draw" arbitrary images by imagemagick commands..."