Bugfix: reference 'rxjs-imports'

File `rxjs-imports.ts` is not referenced anywhere which causes the application fail to start:
```
Uncaught TypeError: action$.ofType(...).concatMap is not a function...
```

Author: mleg

playground/src/index.tsx

@@ -2,6 +2,7 @@ import * as React from 'react';
 import { render } from 'react-dom';
 import { Provider } from 'react-redux';
 
+import './rxjs-imports';
 import store from './store';
 import {
   Home,