title: StackNavigator title not showing in simplest example
body: <p>I just started React Native development, installed <code>Expo</code>, created an app (works), installed <code>react-navigation</code> and tried the first StackNavigator example using the example from <a href="https://reactnavigation.org/docs/intro/" rel="nofollow noreferrer">https://reactnavigation.org/docs/intro/</a>.  I am running <code>npm run ios</code> from the commandline, and using <code>Nuclide</code> IDE. All of which are completely new to me.</p>
  
  <p>The problem is, that on running the example the screen in the iOS emulator shows this:</p>
  
  <p><a href="https://i.stack.imgur.com/uimHE.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/uimHE.png" alt="missing title bar"></a></p>
  
  <p>Instead of showing a title bar with 'Welcome' on it.</p>
  
  <p>As a beginner I have no clue where to go from here. Here's my package.json:</p>
  
  <pre><code>{
    "name": "rnproject",
    "version": "0.1.0",
    "private": true,
    "devDependencies": {
      "babel-cli": "^6.24.1",
      "babel-preset-flow": "^6.23.0",
      "flow-bin": "0.42.0",
      "jest-expo": "~1.0.1",
      "react-native-scripts": "0.0.30",
      "react-test-renderer": "16.0.0-alpha.6"
    },
    "main": "./node_modules/react-native-scripts/build/bin/crna-entry.js",
    "scripts": {
      "start": "react-native-scripts start",
      "eject": "react-native-scripts eject",
      "android": "react-native-scripts android",
      "ios": "react-native-scripts ios",
      "test": "node node_modules/jest/bin/jest.js --watch"
    },
    "jest": {
      "preset": "jest-expo"
    },
    "dependencies": {
      "expo": "^17.0.0",
      "react": "16.0.0-alpha.6",
      "react-native": "^0.44.0",
      "react-navigation": "^1.0.0-beta.11"
    }
  }
  </code></pre>
  
  <p>there's an app.json file with these contents:</p>
  
  <pre><code>{
    "expo": {
      "sdkVersion": "17.0.0"
    }
  } 
  </code></pre>
  
  <p>I also added <code>flow</code>, which throws me no errors in the example code, but 115 errors in the <code>react-navigation</code> package. Most of them look like: <code>identifier 'expect', could not resolve name</code>.</p>