Use latest version of imagemagick

[vinc]

$ identify -version
Version: ImageMagick 6.7.7-10 2017-07-31 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2012 ImageMagick Studio LLC
Features: OpenMP    

There is too much vulnerabilities for older versions of imagemagick: https://www.cvedetails.com/vulnerability-list/vendor_id-1749/Imagemagick.html

Use the latest version: https://github.com/thoughtbot/paperclip/wiki/Upgrade-Imagemagick-On-Heroku