Add `history` to "externals" in production webpack config
Add `history` to "externals" in production webpack config

Since we're using `react-router@2`, there's no reason to bundle
`history` module which is a fallback for `react-router@1`.

As a result, update `webpack/production.config.js` "externals"
to ignore the `history` module.

Production bundle size changes:

| Compression | Before  | After   |
| ----------- | ------- | ------- |
| Unminified  | 78.1 kb | 37.1 kb |
| Minified    | 24 kb   | 8.67 kb |

Where `history` is required in `react-engine`:
- https://github.com/paypal/react-engine/pull/159
- https://github.com/paypal/react-engine/blob/8fd56aa95d9e2ec8ec60c4b542f6a6cce60e91bc/lib/client.js#L62

remarkablemark/react-engine-template,remarkablemark/react-engine-template