Fix broken links in the site

This PR fixes broken links found with [html-proofer](https://github.com/gjtorikian/html-proofer) as suggested in #2593. 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 from new releases blog posts:
````
./_site/news/2014/03/24/jekyll-1-5-0-released/index.html: linking to /docs/history/#150__20140324, but 150__20140324 does not exist
./_site/news/releases/index.html: linking to /docs/history/#150__20140324, but 150__20140324 does not exist
./_site/news/releases/index.html: linking to /docs/history/#103__20130607, but 103__20130607 does not exist
./_site/news/releases/index.html: linking to /docs/history/#102__20130512, but 102__20130512 does not exist
./_site/news/releases/index.html: linking to /docs/history/#101__20130508, but 101__20130508 does not exist
./_site/news/releases/index.html: linking to /docs/history/#100__20130506, but 100__20130506 does not exist
./_site/news/index.html: linking to /docs/history/#150__20140324, but 150__20140324 does not exist
./_site/news/index.html: linking to /docs/history/#103__20130607, but 103__20130607 does not exist
./_site/news/index.html: linking to /docs/history/#102__20130512, but 102__20130512 does not exist
./_site/news/index.html: linking to /docs/history/#101__20130508, but 101__20130508 does not exist
./_site/news/index.html: linking to /docs/history/#100__20130506, but 100__20130506 does not exist
./_site/news/2013/06/07/jekyll-1-0-3-released/index.html: linking to /docs/history/#103__20130607, but 103__20130607 does not exist
./_site/news/2013/05/12/jekyll-1-0-2-released/index.html: linking to /docs/history/#102__20130512, but 102__20130512 does not exist
./_site/news/2013/05/08/jekyll-1-0-1-released/index.html: linking to /docs/history/#101__20130508, but 101__20130508 does not exist
./_site/news/2013/05/05/jekyll-1-0-0-released/index.html: linking to /docs/history/#100__20130506, but 100__20130506 does not exist
````

Running `html-proofer` in the Travis build would also be possible, but that would also mean that an error in the site would cause the whole project to assume a "build failing" status.