Update to jekyll-sitemap v0.6.2

Replaces #95.

5 changes, only 4 affect runtime code:

1. Strip whitespace from output
2. Use `date_to_xmlschema` instead of a custom date format for `lastmod`
3. Create a subclass of `Page` which does not attempt to read from the file system
4. Use `Site#in_source_dir` and `Site#in_dest_dir` per Jekyll v2.5.0 (requires #99)

https://github.com/jekyll/jekyll-sitemap/releases/tag/v0.6.1
https://github.com/jekyll/jekyll-sitemap/releases/tag/v0.6.2
https://github.com/jekyll/jekyll-sitemap/compare/v0.6.0...v0.6.2

:chocolate_bar:

/cc @benbalter @mastahyeti @gregose @ptoomey3 @gjtorikian @spraints