Fix broken links within the site

This PR adds a `.travis.yml` config file to run [html-proofer](https://github.com/gjtorikian/html-proofer) on the site to find broken links as suggested in #2593.

I've also fixed a few of these issues, though I didn't attempt to fix any links to user/contributor content (like missing GitHub users or missing repos), nor any sites that simply timed out (the server might just be down).