External link checker

[alexpearce]

It's easy for links to become stale, see #24. We should automatically check all links in the CI.

The checker does already do some link checking, but it skips external links. I think that Travis CI has access to the Internet, so it should be OK to just curl -I the pages or something.

Downside is: it will break if a site is temporarily down or if the machine the tests are being run on don't have internet access (writing lessons on the 🚄 for example).