Fix Issue with react-jss

Quoting from issue verekia#243:

At the end of chapter 08, I've got some errors just after src/server/render-app.jsx edit:

```
Warning: React.createElement: type is invalid -- expected a string (for built-in components) or a class/function (for composite components) but got: undefined. You likely forgot to export your component from the file it's defined in.
Invariant Violation: Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: undefined. You likely forgot to export your component from the file it's defined in.
    at invariant (/Users/davidfq/Code/js-stack/node_modules/fbjs/lib/invariant.js:44:15)
    at ReactCompositeComponentWrapper.instantiateReactComponent [as _instantiateReactComponent] (/Users/davidfq/Code/js-stack/node_modules/react-dom/lib/instantiateReactComponent.js:74:56)
...
```

It seems that <SheetsRegistryProvider> is not available in react-jss v7.1.0. I've just changed it to <JssProvider> and everything worked.

tutorial/08-bootstrap-jss.md

@@ -373,7 +373,7 @@ export const JSS_SSR_SELECTOR = `.${JSS_SSR_CLASS}`
 - Edit `src/server/render-app.jsx` like so:
 
 ```js
-import { SheetsRegistry, SheetsRegistryProvider } from 'react-jss'
+import { SheetsRegistry, JssProvider } from 'react-jss'
 // [...]
 import { APP_CONTAINER_CLASS, JSS_SSR_CLASS, STATIC_PATH, WDS_PORT } from '../shared/config'
 // [...]
@@ -383,9 +383,9 @@ const renderApp = (location: string, plainPartialState: ?Object, routerContext:
  const appHtml = ReactDOMServer.renderToString(
  <Provider store={store}>
  <StaticRouter location={location} context={routerContext}>
- <SheetsRegistryProvider registry={sheets}>
+ <JssProvider registry={sheets}>
  <App />
- </SheetsRegistryProvider>
+ </JssProvider>
  </StaticRouter>
  </Provider>)
  // [...]