Update from upstream repo facebookincubator/create-react-app@master (#4)

* `react-error-overlay` has no dependencies now (facebook#3263)

* `react-error-overlay` has no dependencies now (it's bundled)

* Use babel 6 for now

* Add external links to deployment services (facebook#3265)

* Add warning when using `react-error-overlay` in production (facebook#3264)

* Add a warning when running minified

* Add more robust check

* Update index.js

* Use production React version for bundled overlay (facebook#3267)

* Use production React version

* We cannot strip our own checks if production

* Keep the sourcemap during minify

* Prevent devtools pollution

* Add some comments

* sigh

* Fix dead link to Jest "expect" docs (facebook#3289)

Closes facebook#3291

packages/react-scripts/template/README.md

@@ -1235,7 +1235,7 @@ it('sums numbers', () => {
 });
 ```
 
-All `expect()` matchers supported by Jest are [extensively documented here](http://facebook.github.io/jest/docs/expect.html).<br>
+All `expect()` matchers supported by Jest are [extensively documented here](https://facebook.github.io/jest/docs/en/expect.html#content).<br>
 You can also use [`jest.fn()` and `expect(fn).toBeCalled()`](http://facebook.github.io/jest/docs/expect.html#tohavebeencalled) to create “spies” or mock functions.
 
 ### Testing Components