Code style update (const, arrow functions)

Created by: tuchk4

Started here - https://github.com/facebookincubator/create-react-app/pull/1433#discussion_r104329257

(only for packages/react-scripts)

• If arrow functions are already used maybe switch others functions to arrow ?
• Same for var -> const. For example:
  • test.js - only const is used
  • start.js - only var is used
  • webpack.config.prod.js both var and constare used