Use new Jekyll configuration for code highlighting

This removes the following warning:

Deprecation: The 'pygments' configuration option has been renamed to 'highlighter'.
Please update your config file accordingly. The allowed values are 'rouge', 'pygments' or null.

_config.yml

@@ -1,6 +1,6 @@
 baseurl: ''
 safe: true
-pygments: true
+highlighter: pygments
 lsi: false
 markdown: redcarpet
 exclude: