Code block styles no longer syntax highlighted

[benkeen]

Hi! We updated our Storybook from 5.0.11 to 5.2.4 and even though this module still works, any code blocks within our md files are no longer syntax highlighted.

e.g. in our MD file we have:

```javascript
import { something } from './Something';
(three ` here to close)

Before the Storybook update, the generated HTML from storybook-readme looked like:

<pre class=" language-javascript"><code class=" language-javascript">[code here]</code></pre>

Afterwards it looked the same, except the top-level pre tag was missing the class:

<pre><code class=" language-javascript">[code here]</code></pre>

Also, adding that class name back didn't fix the styling issue. It looks like maybe the CSS isn't being imported any more?

Thanks, great project!