chore: add Travis CI docs deployment job and update build dependencies

- Add automatic docs deployment stage for master branch pushes in .travis.yml
- Include python3-markdown, python3-pygments, python3-setuptools, and ruby-sass as build dependencies
- Switch from container-based to sudo-required builds with trusty dist for custom Pygments lexer installation
- Ensure build directory exists before generating docs in Makefile

Makefile

@@ -90,6 +90,7 @@ unit_test:
 
 # Generate the Wren site.
 docs:
+	mkdir -p build
 	$(V) ./util/generate_docs.py
 
 # Continuously generate and serve the Wren site.