https://llvm.org/ link checks fail on travis

[oli-obk]

I'm assuming it's some sort of rate limiting or general overload protection on their side. Maybe we should just whitelist it?

[mark-i-m]

There were also some failures a while back with the ACM digital library #249. And I saw a few with a link to discord.gg. They always seem to be transient...

[shepmaster]

In a magical world, the tests could allow small amounts of errors in a time period, but warn if there's a consistent pattern. Sounds hard though.

[mark-i-m]

Yes, that sounds pretty hard. Also, I find that the errors go away after an hour or so...

[mark-i-m]

This has been happening a bit more frequently as we add more links. Sometimes it is actually the remote responding slowly (e.g. its slow from my browser too), but I think usually it is travis throttling us or something...

[mark-i-m]

We haven't had as many problems with this since adopting mdbook-linkcheck's new caching scheme, so I will go ahead and close this.