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).

There also seemed to have been an issue with some anchors linking to the History page (this is evident in the results below).

Here's the results on the Travis build that ran on the last commit in my branch: https://travis-ci.org/alfredxing/jekyll/builds/29934452