remove jekyll-sitemap pull from github master

Pull from rubygems.org instead

Author: staeiou

Gemfile

@@ -22,5 +22,5 @@ gem "wdm", "~> 0.1.0" if Gem.win_platform?
 group :jekyll_plugins do
   # gem "jekyll-archives"
   gem "jekyll-feed"
-  gem 'jekyll-sitemap', :github => 'jekyll/jekyll-sitemap', :branch => 'master'
+  gem 'jekyll-sitemap'
 end