Update to jekyll-sitemap v0.6.1

3 changes, only 2 affect runtime code:

1. Strip whitespace from output
2. Use `date_to_xmlschema` instead of a custom date format for `lastmod`

https://github.com/jekyll/jekyll-sitemap/releases/tag/v0.6.1
https://github.com/jekyll/jekyll-sitemap/compare/v0.6.0...v0.6.1

:chocolate_bar:

/cc @benbalter