upgrade to react-router v4

[travi]

• handles hot-reloading of async-routes
  • https://github.com/gaearon/react-hot-loader/tree/460b39a155c58919706546f62ea5b918a2bdb02f#known-limitations
  • https://github.com/ReactTraining/react-router/blob/v4/README.md#v4-faq
• does server-rendering need to change? https://ebaytech.berlin/universal-web-apps-with-react-router-4-15002bb30ccb#.7mipe4a0i
  • since i use redial, i don't think it should be needed