new htmlproofer failures

seen at e.g. https://travis-ci.org/scala/scala-lang/builds/405576488

```
- ./_site/documentation/your-first-lines-of-scala.html
  *  External link http://java.sun.com/javase/ failed: 403 No error
- ./_site/news/2.12.0/index.html
  *  External link http://www.oracle.com/technetwork/java/javase/downloads/index.html failed: 403 No error
```