Don't test external links in html-proofer for now:
Don't test external links in html-proofer for now:

- It's not in CI now: You should run by hand
  $ bundle exec rake test
- There are already 2804 errors for internal links
- Maybe we should investigate it first, then including external links

yasslab/railsguides.jp,yasslab/railsguides.jp,yasslab/railsguides.jp,yasslab/railsguides.jp