youtube-dl: Update net/youtube-dl to 20180908

Changes:
version 2018.09.08

Extractors
* [youtube] Fix extraction (#17457, #17464)
+ [pornhub:uservideos] Add support for new URLs (#17388)
* [iprima] Confirm adult check (#17437)
* [slideslive] Make check for video service name case-insensitive (#17429)
* [radiojavan] Fix extraction (#17151)
* [generic] Skip unsuccessful jwplayer extraction (#16735)

(leot)