[paperclip] An error was received while processing: #<Paperclip::Errors::NotIdentifiedByImageMagickError: Paperclip::Errors::NotIdentifiedByImageMagickError>
ローカルに ghostscript が入っていなかった!
brew install ghostscript
# https://www.ghostscript.com/
で解決。
http://stackoverflow.com/questions/15744843/paperclip-error-notidentifiedbyimagemagickerror-when-scaling-images