Fix Rouge wrapping

nanoc wraps with `<pre><code`. Rouge wraps with `<pre><code`. The end result is too much wrapping:

```html
<pre><code class="language-ruby"><pre><code class="highlight">
```

This PR introduces some HTML manipulation that merges the `pre`s and `code`s:

```html
<pre><code class="language-ruby highlight">
```

The tests in nanoc master only started failing after a recent Rouge upgrade.

CC @gjtorikian @bobthecow