title: rake assets:precompile failing during push to Heroku
body: <p>I'm currently using asset_sync in my Rails app, and I have the environment variables set in my Heroku app. When I run heroku config I get:</p>
  
  <pre><code>AWS_ACCESS_KEY_ID:     XXXXXXXXXXXXXXXXXX
  AWS_SECRET_ACCESS_KEY: XXXXXXXXXXXXXXXXXX
  FOG_DIRECTORY:         MY-BUCKET-NAME
  FOG_PROVIDER:          AWS
  etc...
  </code></pre>
  
  <p>When I push my app to Heroku, it tries to run rake assets:precompile and I get the following message:</p>
  
  <pre><code>Preparing app for Rails asset pipeline
  Running: rake assets:precompile
  /usr/local/bin/ruby /tmp/build_2pa7aisux9av8/vendor/bundle/ruby/1.9.1/bin/rake assets:precompile:nondigest RAILS_ENV=production RAILS_GROUPS=assets
  AssetSync: using /tmp/build_2pa7aisux9av8/config/initializers/asset_sync.rb
  rake aborted!
  Fog directory can't be blank, Aws access key can't be blank, Aws secret access key can't be blank
  </code></pre>
  
  <p>But then I run:</p>
  
  <pre><code>heroku run rake assets:precompile --app my-app-name
  </code></pre>
  
  <p>...and it processes everything and syncs to S3 just fine:</p>
  
  <pre><code>Running `rake assets:precompile` attached to terminal... up, run.1
  /usr/local/bin/ruby /app/vendor/bundle/ruby/1.9.1/bin/rake assets:precompile:all RAILS_ENV=staging RAILS_GROUPS=assets
  AssetSync: using /app/config/initializers/asset_sync.rb
  /usr/local/bin/ruby /app/vendor/bundle/ruby/1.9.1/bin/rake assets:precompile:nondigest RAILS_ENV=staging RAILS_GROUPS=assets
  AssetSync: using /app/config/initializers/asset_sync.rb
  AssetSync: Syncing.
  Using: Directory Search of /app/public/assets
  Uploading: assets/application-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX.css
  Uploading: assets/application-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX.css.gz
  Uploading: assets/application-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX.css.gz
  Uploading: assets/application-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX.css
  AssetSync: Done.
  </code></pre>
  
  <p>Any ideas why it wouldn't work during the push but it would work fine when I heroku run rake assets:precompile?</p>