Highlighter

I am running jeykll 2.3.0 and I get this warning when running `jeykll serve -w`:
```
$ jekyll serve -w
Configuration file: /home/username/Jekyll/blueberry/_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: /home/username/Jekyll/blueberry
       Destination: /home/username/Jekyll/blueberry/_site
      Generating... 
                    done.
 Auto-regeneration: enabled for '/home/username/Jekyll/blueberry'
Configuration file: /home/username/Jekyll/blueberry/_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.
    Server address: http://0.0.0.0:4000/

````

After making the change of `pygments: true` to `highlighter: pygments` in `config.yml` it ran without a warning

```
$ jekyll serve -w
Configuration file: /home/username/Jekyll/blueberry/_config.yml
            Source: /home/username/Jekyll/blueberry
       Destination: /home/username/Jekyll/blueberry/_site
      Generating... 
                    done.
 Auto-regeneration: enabled for '/home/username/Jekyll/blueberry'
Configuration file: /home/username/Jekyll/blueberry/_config.yml
    Server address: http://0.0.0.0:4000/
  Server running... press ctrl-c to stop.
```