Ignore www.oracle.com in htmlproofer

[ashawley]

These links can't be helped. They just keep failing intermittently.

If these break, I'm pretty sure we'll hear about it, unlike a lot of the many other obscure links on the site. I believe it only affects 3 links on 3 pages:

- ./_site/documentation/your-first-lines-of-scala.html
  *  External link https://www.oracle.com/technetwork/java/index.html failed: 403 No error
- ./_site/download/2.12.7.html
  *  External link http://www.oracle.com/technetwork/java/javase/downloads/index.html failed: 403 No error
- ./_site/news/2.12.0/index.html
  *  External link https://www.oracle.com/technetwork/java/javase/downloads/index.html failed: 403 No error

Fixes #945.

[Philippus]

agree, we've spent more than enough time on this :)