'pygments' option now called 'highlighter'

Since pygments is not the only option you can use to do code highlighting anymore that option is now called 'highlighter'. You can either give it the value `pygments` or the value `rouge`. 

I'd recommend using `rouge` (https://github.com/jneen/rouge) because it is written in Ruby and spares you the pain of installing Python on your system just to do code highlighting in Jekyll.