Fix the deprecated pygments option to highlighter

Since Jekyll 3.0 the pygments option was deprecated. Instead we
must use highlighter option and put pygments value in order to
continue to use pygments as syntax highlighting library [1].

[1] https://jekyllrb.com/docs/upgrading/2-to-3/#syntax-highlighter-changed

_config.yml

@@ -9,7 +9,7 @@ redcarpet:
     extensions: [smart, tables]
 
 # colorize code snippets with the pygments module
-pygments: true
+highlighter: pygments
 
 # The path structure for blog posts.
 permalink: /blog/:year/:month/:day/:title.html