Update 'pygments' configuration options

[livlab]

After cloning this repo for the first time, I got the following message after running jekyll serve --watch:

Configuration file: /nprapps.github.com/_config.yml
       Deprecation: The 'pygments' configuration option has been renamed to 'highlighter'. Please update your config file accordingly. The allowed values are 'rouge', 'pygments' or null.
            Source: /nprapps.github.com
       Destination: //nprapps.github.com/_site
      Generating...
                    done.
 Auto-regeneration: enabled for '/nprapps.github.com'
Configuration file: /nprapps.github.com/_config.yml
       Deprecation: The 'pygments' configuration option has been renamed to 'highlighter'. Please update your config file accordingly. The allowed values are 'rouge', 'pygments' or null.

I'm happy to help with it if you can guide me on where the change needs to be made. Is the change really needed?

[inadarei]

This is not a critical warning, everything should still work, but Jekyll has updated its syntax for code highlighting. This pull request should "fix" the "problem": #47

[livlab]

Thanks! 👍