Rails 3 assets not compressed

My assets are not being compressed. I'm using 3.2.18, with asset group having these gems :

group :assets do
  gem "compass-rails"
  gem 'coffee-rails', '~> 3.2.1'
  gem 'elementaljs-rails'
  gem 'i18n-js'
  gem 'tilt',   '~> 1.4.1'
  gem 'sass-rails',   '~> 3.2.3'
  gem 'turbo-sprockets-rails3'
  gem 'uglifier', '>= 1.0.3'
  gem 'bootstrap-sass', '~> 3.1.1'
  gem 'ejs'
  gem 'jquery-ui-rails'
  gem 'angular-ui-sortable-rails', github: "sherwyngoh/angular-ui-sortable-rails"
end

This is my assets related production config :

config.serve_static_assets = false
  config.assets.js_compressor = :uglifier
  config.assets.css_compressor = :sass
  config.assets.compile = false
  config.assets.digest = true

When I do RAILS_EVN=production rake assets:precompile, and take a look at public/assets/application-xxx.css I see the regular css file like I do when running server in development. Why is not getting compressed?