Fix rendering of multiple emoji in code blocks

Fixes https://github.com/atom/markdown-preview/issues/84

This modifies the loop which replaces emojis in code blocks so that only the element that should be changed (`this`) is selected.

Before (notice the duplicates in the preview window):

![screen shot 2014-05-20 at 9 18 16 pm](https://cloud.githubusercontent.com/assets/38924/3032082/89a5a59e-e053-11e3-95e7-c4274cb5d7e4.png)

Failing test:

```
roaster
  emoji
    it does not mess up coded emoji
      Expected '<pre><code class="lang-ruby">not :trollface:
</code></pre>
<p>wow <code>that is nice :smiley:</code></p>
<p><code>:smiley:</code></p>
<p><code>:smiley:</code></p>
<pre><code>:trollface:
</code></pre><pre><code>:trollface:
</code></pre>' to equal '<pre><code class="lang-ruby">not :trollface:
</code></pre>
<p>wow <code>that is nice :smiley:</code></p>
<p><code>:laughing:</code></p>
<p><code>:lipstick:</code></p>
<pre><code>:laughing:
</code></pre><pre><code>:lipstick:
</code></pre>'. (test/roaster-spec.coffee:126:35)
```

After (no duplicates!):

![screen shot 2014-05-19 at 9 09 53 pm](https://cloud.githubusercontent.com/assets/38924/3032012/dee3435a-e052-11e3-8e9c-e35031841bd2.png)

cc @gjtorikian