Use sprockets-es6 to handle jsx

sprockets.rb

# config/initializers/sprockets.rb
module Sprockets
  if respond_to?(:register_transformer)
    register_mime_type 'text/jsx', extensions: ['.jsx'], charset: :unicode
    register_transformer 'text/jsx', 'application/javascript', ::BabelTransformer
    register_preprocessor 'text/jsx', DirectiveProcessor
  end

  if respond_to?(:register_engine)
    args = ['.jsx', ::BabelTransformer]
    args << { mime_type: 'text/jsx', silence_deprecation: true } if Sprockets::VERSION.start_with?("3")
    register_engine(*args)
  end
end