Updating highlight configuration to use pygments

Jekyll no longer uses ```pygments: true```as a configuration option. The new way to do this is by using ```highlighter: pygments``` to use pygments.

Starting the jekyll server this will will display the following warning message:
```
Deprecation: The 'pygments' configuration option has been renamed to 'highlighter'. Please update your config file accordingly. The allowed values are 'rouge', 'pygments' or null.
```