Update dependencies

 * github-markup 1.3.0
 * sanitize 3.0.2

I tried to update `rouge` but they made some changes.
Code block are now using `<code>` instead of `<pre>` and as a result 13 tests are failing:

```
  "<pre class=\"highlight\"><span class=\"s1\">'hi'</span></pre>" expected to be HTML equivalent to "<pre><code class=\"highlight\"><span class=\"s1\">'hi'</span></code></pre>\n\n".
```

Ref #93