Update config for Jekyll 2.0
Change code highlighting in all posts.

Use jekyll highlight tags for code syntax highlighting.
Kramdown fenced code blocks did not render correctly in github-pages.

This is not ideal as I wanted to avoid any templating language in the
posts themselves, but I could not get fenced code blocks working.
I tried coderay and the GFM input option to Kramdown yet it never
rendered correctly.