scalaGH-644: Remove static html styling of spec code blocks

Prior to this Jekyll configuration change code blocks were being marked up
in a ways that disrupted MathJax styling.

Tested as far a possible locally. Confidence mainly drawn from html comparison
to working 2.11.x version.

Author: janekdb

spec/_config.yml

@@ -1,7 +1,7 @@
 baseurl: /files/archive/spec/2.12
 safe: true
 lsi: false
-highlighter: null
+highlighter: false
 markdown: redcarpet
 encoding: utf-8
 redcarpet: