s/pygments/highlighter/ to be rid of Deprecation warnings in jekyll

`make server` is generating the following output:

```
Configuration file: /Users/srawlins/code/dartlang.org/src/site/_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: .
       Destination: ../../build/static
      Generating...
```

This PR addresses that deprecation warning.
I think it just needs to be falsey. But I bumped it to null instead.