jpegrecompress worker: `jpeg-recompress` not found; - ruby-on-rails

Question

I recently installed the paperclip-optimizer gem in my Rails 4 app on Heroku. Whenever I attempt to save an uploaded image attachment, I get the following error: jpegrecompress worker: 'jpeg-recompress' not found; in my logs from Heroku, but not on localhost.
I'm not sure what I'm missing. I can't seem to find a single StackOverflow post, blog post, or other resource on this problem.
Here is my config/initializers/paperclip_optimizer.rb:
Paperclip::PaperclipOptimizer.default_options = {
skip_missing_workers: true,
advpng: false,
gifsicle: false,
jhead: false,
jpegoptim: false,
jpegrecompress: true,
jpegtran: false,
optipng: false,
pngcrush: false,
pngout: false,
pngquant: true,
svgo: false,
nice: 10, # Nice level (defaults to 10)
threads: 1, # Number of threads or disable (defaults to number of processors)
verbose: false, # Verbose output (defaults to false)
pack: nil, # Require image_optim_pack or disable it, by default image_optim_pack will be used if available,
allow_lossy: true, # Allow lossy workers and optimizations (defaults to false)
jpegrecompress: {
quality: 2 # JPEG quality preset: 0 - low, 1 - medium, 2 - high, 3 - veryhigh (defaults to 3)
},
pngquant: {
quality: 100..100, # min..max - don't save below min, use less colors below max (both in range 0..100; in yaml - !ruby/range 0..100) (defaults to 100..100)
speed: 3 # speed/quality trade-off: 1 - slow, 3 - default, 11 - fast & rough (defaults to 3)
}
}
In my photo.rb model I have:
has_attached_file :image, styles: { large: "600x600>", medium: "300x300>", thumb: "100x100>" }, default_url: "", processors: [:thumbnail, :paperclip_optimizer]
In my .buildpacks file for Heroku I have:
https://github.com/ddollar/heroku-buildpack-multi.git
https://github.com/heroku/heroku-buildpack-ruby.git
https://github.com/bobbus/image-optim-buildpack.git
I added the Ruby buildpack to solve this problem I was having earlier, but now it seems as though the multi and the image-optim buildpacks aren't loading.
Thanks!

Answer 1

After much head banging and keyboard bashing, I figured out how to solve the problem. In my case, I'm running a Rails 4.0 app on the Cedar-14 stack running on the Puma webserver, so these instructions may or may not be of use to you if you're configuration is different.
I followed the steps on this Heroku Dev Center article on adding multiple buildpacks to an app, which is now supported by Heroku.
In my case, I have the following buildpacks added for my app:
=== staging-app Buildpack URLs
1. https://github.com/bobbus/image-optim-buildpack.git
2. https://github.com/heroku/heroku-buildpack-ruby.git
The order is critical. The last buildpack on the list is the first one to be loaded. In my case, I need the Ruby buildpack so that the bundle command in my Procfile is recognized. See this question for more on that issue.
Do NOT use the Heroku-Buildpack-Multi. From what I understand, this is the old way to support multiple buildpacks. Adding this will cause your pushes to be rejected.
In my Gemfile, I needed to include the image_optim and image_optim_pack gems. These gems make the appropriate binaries available on OS X and Linux environments, from what I understand. It is very important to place these gems before paperclip-optimizer in the Gemfile, like so
gem 'paperclip', '~> 4.1'
group :production do
# Needs to be placed before paperclip-optimizer
gem 'image_optim'
gem 'image_optim_pack'
end
gem 'paperclip-optimizer'
Configure your paperclip-optimizer settings as you see fit and push to production.
Hopefully this helps someone in my situation.