Fixes this exception caused by the animated_gif autotagger trying to access the
image file, which fails if the image is not hosted on the webserver.
Magick::ImageMagickError exception raised
unable to open image `/var/www/danbooru2/releases/20170322212609/public/data/f0e32c0e2d906c4932858866346f98b5.gif': No such file or directory @ error/blob.c/OpenBlob/2712
app/models/post.rb:177:in `ping'
app/models/post.rb:177:in `is_animated_gif?'
app/models/post.rb:684:in `add_automatic_tags'
app/models/post.rb:638:in `normalize_tags'
app/controllers/posts_controller.rb:49:in `update'