Use the cache and the new build env
Merge branch 'master' of github.com:jekyll/jekyll-sitemap

* 'master' of github.com:jekyll/jekyll-sitemap:
  Release :gem: v0.6.1
  Update history to reflect merge of #40 [ci skip]
  Update history to reflect merge of #49 [ci skip]
  Update test for new date format
  Fixed date
  Wrong number
  Add issue with sitemap plugin
  Strip extra whitespace from output