Update jekyll-sitemap to 0.3.0

Still compatible with Jekyll 1.5.1.

[Changes in jekyll-sitemap at 0.3.0](https://github.com/jekyll/jekyll-sitemap/releases/tag/v0.3.0):

  * Generate sitemap using `html_pages` (https://github.com/jekyll/jekyll-sitemap/issues/10)
  * Remove stray `sitemap.xsl` from template (https://github.com/jekyll/jekyll-sitemap/issues/8)
  * Added Travis (https://github.com/jekyll/jekyll-sitemap/issues/6)
  * Better timezone support in tests (https://github.com/jekyll/jekyll-sitemap/issues/7)