Bump jekyll-sitemap version

Jekyll-sitemap has released a 0.5.0 version:
http://rubygems.org/gems/jekyll-sitemap

The best feature of the new version is you can now exclude sites from the sitemap. This comes in handy for custom 404 pages on Github:
```
---
title: 404
layout: default
permalink: /404.html
sitemap: false
---
```

Here is the corresponding code: https://github.com/jekyll/jekyll-sitemap/blob/v0.5.0/lib/sitemap.xml#L11

All changes:
https://github.com/jekyll/jekyll-sitemap/compare/v0.3.0...v0.5.0

Changelog:
https://github.com/jekyll/jekyll-sitemap/blob/v0.5.0/History.markdown