Load gems of assets group before of precompile

lib/sprockets/rails/task.rb

@@ -12,6 +12,8 @@ def define
           # Override this task change the loaded dependencies
           desc "Load asset compile environment"
           task :environment do
+            # Load gems in assets group of Gemfile
+            Bundler.require(:assets) if defined?(Bundler)
             # Load full Rails environment by default
             Rake::Task['environment'].invoke
           end