title: IOS react-navigation: screen not rerendered with TabNavigator
body: <p>There are screens Home and New. Home displays a list of data stored in a db. New writes to db. Home screen should remount to display new data when user navigates to it but doesn't. Is this expected behavior, and is there a simple fix? </p>
  
  <p>Home Screen:</p>
  
  <pre><code>class Home extends React.Component {
  static navigationOptions = {
      title: 'Home',
  };
  
  constructor(props) {
      super(props);
  
      realm = new Realm({schema: [Case]});
      this.state = {
          cases: realm.objects('Case'),
      }
  };
  
  render() {
      const listItems = this.state.cases.map((data, index) =&gt;
          &lt;View style={Styles.listItem}&gt;
              &lt;Text key={index}&gt;
                  {data.name}
              &lt;/Text&gt;
          &lt;/View&gt;
      );
      return (
          &lt;View&gt;
              &lt;ScrollView&gt;
                  { listItems }
              &lt;/ScrollView&gt;
          &lt;/View&gt;
      );
  }
  </code></pre>
  
  <p>Routing Configs:</p>
  
  <pre><code>const MainScreenNavigator = TabNavigator({
  Home: { screen: Home },
  New: { screen: NewCase },
  });
  
  const SimpleApp = StackNavigator({
      Home: { screen: MainScreenNavigator },
  });
  
  AppRegistry.registerComponent('SimpleApp', () =&gt; SimpleApp);
  </code></pre>