some new 404s are causing PR validation to fail

https://travis-ci.org/github/scala/scala-lang/builds/743033420

```
- ./_site/pastevents/index.html
*  External link http://www.devoxx.com/ failed: 404 No error
*  External link https://sphere.it/sphere-it-2019-edition/ failed: 404 No error
```