title: React jest shallowRender getRenderOutput returning null
body: <p>I've been studying React, and I tried to make a jest test with the shallowRender but always return null on shallowRenderer.getRenderOutput()</p>
  
  <p><strong>MyComponent.react.js</strong></p>
  
  <pre><code>import React from 'react';
  
  export default class MyComponent extends React.Component {
    render() {
      return &lt;div&gt;Hello World!&lt;/div&gt;;
    }
  }
  </code></pre>
  
  <p><strong>__test__/MyComponent.react.js</strong></p>
  
  <pre><code>jest.dontMock('../MyComponent.react.js');
  
  import React from 'react/addons';
  import MyComponent from '../MyComponent.react.js';
  
  var TestUtils = React.addons.TestUtils;
  
  describe('MyComponent', () =&gt; {
  
    it('test', () =&gt; {
      let shallowRenderer = TestUtils.createRenderer();
      shallowRenderer.render(&lt;MyComponent/&gt;);
      let vdom = shallowRenderer.getRenderOutput();
  
      console.log(vdom);
      expect(1).toEqual(1);
    });
  });
  </code></pre>
  
  <p><strong>npm test</strong></p>
  
  <pre><code>&gt; jest-test@1.0.0 test /home/bruno/Stuffs/jest-test
  &gt; jest
  
  Using Jest CLI v0.5.8
   PASS  __tests__/MyComponent.js (0.897s)
  null
  1 test passed (1 total)
  Run time: 1.189s
  </code></pre>
  
  <p><strong>package.json</strong></p>
  
  <pre><code>{
    ...
    "scripts": {
      "test": "jest"
    },
    "devDependencies": {
      "babel": "^5.8.23",
      "babel-jest": "^5.3.0",
      "jest-cli": "^0.5.8",
      "react": "^0.13.3",
      "skin-deep": "^0.6.1"
    },
    "jest": {
      "scriptPreprocessor": "&lt;rootDir&gt;/node_modules/babel-jest",
      "testFileExtensions": [
        "es6",
        "js"
      ],
      "unmockedModulePathPatterns": [
        "&lt;rootDir&gt;/node_modules/react"
      ],
      "moduleFileExtensions": [
        "js",
        "json",
        "es6"
      ]
    }
  }
  </code></pre>