title: firebase undefined when working with ReactReduxFirebaseProvider
body: <p>Im trying to migrate from react-redux v2 to v3 and start to use the ReactReduxFirebaseProvider. But I don't know why I get:</p>
  
  <pre><code>Line 22:  'firebase' is not defined  no-undef
  </code></pre>
  
  <p><strong>This is my code:</strong></p>
  
  <pre><code>import React from 'react';
  import ReactDOM from 'react-dom';
  import './index.css';
  import App from './App';
  import registerServiceWorker from './registerServiceWorker';
  import { createStore, applyMiddleware, compose } from 'redux'
  import rootReducer from './store/reducers/rootReducer'
  import { Provider } from 'react-redux'
  import thunk from 'redux-thunk'
  import { createFirestoreInstance, getFirestore } from 'redux-firestore'
  import { ReactReduxFirebaseProvider, getFirebase } from 'react-redux-firebase'
  import fbConfig from './config/fbConfig'
  
  
  const store = createStore(rootReducer,
    compose(
      applyMiddleware(thunk.withExtraArgument({ getFirebase, getFirestore }))
    )
  );
  
  const rrfProps = {
    firebase,
    config: fbConfig,
    dispatch: store.dispatch,
    createFirestoreInstance
  }
  
  ReactDOM.render(
    &lt;Provider store={store}&gt;
      &lt;ReactReduxFirebaseProvider {...rrfProps}&gt;
        &lt;App /&gt;
      &lt;/ReactReduxFirebaseProvider&gt;
    &lt;/Provider&gt;, document.getElementById('root'));
  registerServiceWorker();
  </code></pre>
  
  <p>Can someone point me in the right direction of what I am doing wrong?</p>