title: Import is undefined in Jest snapshot test
body: <p>I'm using Jest and Enzyme to snapshot test a component that uses <code>CSSTransition</code> from <a href="https://github.com/reactjs/react-transition-group" rel="nofollow noreferrer"><code>react-transition-group</code></a> (v2.1.0). When I create a snapshot of the component, <code>CSSTransition</code> appears <code>undefined</code>. This causes React to throw a warning:</p>
  
  <pre><code>Warning: React.createElement: type is invalid -- expected a string 
    (for built-in components) or a class/function (for composite components) but got:
    undefined. You likely forgot to export your component
    from the file it's defined in.
  </code></pre>
  
  <p>Here is the broken snapshot diff:</p>
  
  <pre><code>-  &lt;CSSTransitionGroup
  -    transitionAppear={false}
  -    transitionEnter={true}
  -    transitionEnterTimeout={150}
  -    transitionLeave={true}
  -    transitionLeaveTimeout={150}
  -    transitionName="fade"
  -  /&gt;
  +  &lt;undefined
  +    classNames="fade"
  +    in={false}
  +    mountOnEnter={true}
  +    timeout={150}
  +    unmountOnExit={true}
  +  &gt;
  +    &lt;div
  +      className="shadow"
  +      onClick={[Function]}
  +    /&gt;
  +  &lt;/undefined&gt;
  </code></pre>
  
  <p>The error first appeared when I upgraded from react-transition-group v1.2.0 to v2.1.0. Here's the relevant part of the commit that first caused the break to appear:</p>
  
  <pre><code>-import { CSSTransitionGroup } from 'react-transition-group';
  +import CSSTransition from 'react-transition-group/CSSTransition';
  
  -      &lt;CSSTransitionGroup
  -        transitionName='fade'
  -        transitionEnterTimeout={150}
  -        transitionLeaveTimeout={150}
  +      &lt;CSSTransition
  +        in={!!this.state.selectedCategory}
  +        classNames='fade'
  +        timeout={150}
  +        mountOnEnter
  +        unmountOnExit
         &gt;
  -      { this.state.selectedCategory &amp;&amp;
           &lt;div key='shadow' className='shadow' onClick={this.handleCancel} /&gt;
  -      }
  -      &lt;/CSSTransitionGroup&gt;
  +      &lt;/CSSTransition&gt;
  </code></pre>
  
  <p>Anyone have any ideas what's causing this?</p>