development.rb

config.assets.compress = false

Gemfile

group :assets do
  gem 'sass-rails',   '~> 4.0.0.beta1'
  gem 'coffee-rails', '~> 4.0.0.beta1'
  gem 'sprockets-rails', :require => 'sprockets/railtie'
  # See https://github.com/sstephenson/execjs#readme for more supported runtimes
  # gem 'therubyracer', platforms: :ruby

  gem 'uglifier', '>= 1.0.3'
end

production.rb

# Compress JavaScripts and CSS
config.assets.compress = true

config.assets.compile = true

# Generate digests for assets URLs
config.assets.digest = true

config.assets.configure do |env|
  env.js_compressor  = :uglify # or :closure, :yui
  env.css_compressor = :sass   # or :yui

  env.logger = Rails.logger

  env.cache = ActiveSupport::Cache::FileStore.new("tmp/cache/assets")
end