Install pygments gem during web deployment

[samcowger]

Though my local build of the tutorial has C syntax highlighting, the deployed version does not. Looking at the logs of a recent deployment, I see the following:

<...>
Create build/tutorial.adoc
cp -r src/images build/images
asciidoctor --doctype book build/tutorial.adoc -o build/tutorial.html
asciidoctor: WARNING: optional gem 'pygments.rb' is not available (reason: cannot load 'pygments'). Functionality disabled.
cd build; zip -r exercises.zip exercises > /dev/null

I tried making this change and running the publishing workflow on my own fork, and it worked there, so I certainly hope it will work here as well.