Enable google_prettify

Hello,
I found google_prettify dosen't work when I use ruhoh 2.1.1. After working on this issue for a while, I was able to find out that the reason was caused by the commit [Move google_prettify](https://github.com/ruhoh/ruhoh.rb/commit/fdeff636da2ec2deb3ce239651c2e52da7883df7#system/widgets) at the project ruhoh.rb after Moving google_prettify into a subdirectory to allow for a unified syntax highlighting widget. 

After the change by that commit, the name of the widget was changed. However, the corresponding references in `config.yml` and the `layout/default.html` are not updated synchronously. 

So I fix the problem to enable google_prettify again.