Getting message about missing multi_json-1.3.6 when Capistrano tries to run assets:precompile
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I'm using Capistrano with Rails 3.2.1. When I do `cap deploy`, I get this error:

    Could not find multi_json-1.3.6 in any of the sources

Here's some of the relevant output leading up to this error:

      * executing `deploy:assets:precompile'
      * executing "cd /home/jason/dittypad-cap/releases/20120728190221 && bundle exec rake RAILS_ENV=production RAILS_GROUPS=assets assets:precompile"
        servers: ["sniphq.com"]
        [sniphq.com] executing command
     ** [out :: sniphq.com] Could not find multi_json-1.3.6 in any of the sources

I do have this gem, though:

    $ bundle list | grep multi_json
      * multi_json (1.3.6)

I was curious to see what would happen if I ran `assets:precompile` directly in production. The first time I tried it, it worked. But now when I try it I get this:

    $ bundle exec rake RAILS_ENV=production RAILS_GROUPS=assets assets:precompile
    /home/jason/.rvm/gems/ruby-1.9.2-p290/gems/bundler-1.0.22/lib/bundler/rubygems_integration.rb:143:in `block in replace_gem': rake is not part of the bundle. Add it to Gemfile. (Gem::LoadError)
    	from /home/jason/dittypad-cap/shared/bundle/ruby/1.9.1/bin/rake:18:in `<main>'

Why is it complaining about `multi_json` being missing even though I have it?