Checking for broken links with TravisCI

See #19 for fixing all the existing broken links.

Once that's done, we should add a link checker to TravisCI to keep appraised of any broken links (due to external site changes) or bad links in incoming pull request.

@JoaoRodrigues suggested following https://www.jacobtomlinson.co.uk/jekyll/2015/02/18/test-you-jekyll-blog-with-travis-ci/ which suggests this for the `.travis.yml` file:

``` yaml
language: ruby
rvm:
- 2.1
script:
- bundle exec jekyll build
- bundle exec htmlproof ./_site --only-4xx --check-favicon --check-html
env:
global:
- NOKOGIRI_USE_SYSTEM_LIBRARIES=true # speeds up installation of html-proofer
```