Remove pygments highlighter

[ryanfb]

From GitHub:

You are attempting to use the 'pygments' highlighter, which is currently unsupported on GitHub Pages. Your site will use 'rouge' for highlighting instead. To suppress this warning, change the 'highlighter' value to 'rouge' in your '_config.yml' and ensure the 'pygments' key is unset. For more information, see https://help.github.com/articles/page-build-failed-config-file-error/#fixing-highlighting-errors.