title: Can't find variable: Navigate with react-navigation
body: <p>i am having this simple code in the same index.android.js file:</p>
  
  <pre><code>import React, { Component } from 'react';
  import {
    AppRegistry,
    Text,
    View,
    Button
  } from 'react-native';
  import {StackNavigator} from 'react-navigation';
  
  // First Page
  class FirstPage extends Component {
    static navigationOptions = {
      title: 'Welcome',
    };
  
    render() {
      const { navigate } = this.props.navigation;
      return (
        &lt;View&gt;
          &lt;Text&gt;Hello First Page&lt;/Text&gt;
          &lt;Button 
            onPress={() =&gt; navigate('SecondPage')}
            title="Go to second page"
          /&gt;
        &lt;/View&gt;
      );
    }
  }
  
  // Second Page
  class SecondPage extends Component {
    static navigationOptions = {
      title: 'Second Page',
    };
  
    render() {
      const { navigate } = this.props.navigation;
      return (
        &lt;View&gt;
          &lt;Text&gt;Hello Second Page&lt;/Text&gt;
          &lt;Button 
            onPress={() =&gt; navigate('FirstPage')}
            title="Go to first page"
          /&gt;
        &lt;/View&gt;
      );
    }
  }
  
  
  const SimpleApp = StackNavigator({
    FirstPage: { screen: FirstPage},
    SecondPage: { screen: SecondPage},
  });
  
  AppRegistry.registerComponent('MoviesTickets_YCH', () =&gt; FirstPage);
  </code></pre>
  
  <p>All i want is to navigate between the screens, but i have the error message: can't find variable: navigate, i have no clue why it happens, any idea how?</p>
  
  <p>Thanks for your help</p>