Turn on syntax highlighting of code blocks

According to Github Pages guides, the `highlighter` option
should be set to `pygments` (which is the default), see
<https://help.github.com/articles/using-jekyll-with-pages/#defaults>

By overriding it with `true`, we were effectively disabling
syntax highlighting.

Author: Alex Jahraus

_config.yml

@@ -1,6 +1,6 @@
 safe: true
 lsi: false
-highlighter: true
+highlighter: pygments
 future: true
 markdown: redcarpet
 encoding: utf-8