youtube-dl: Update to 20190407

Changes:
version 2019.04.07

Core
+ [downloader/external] Pass rtmp_conn to ffmpeg

Extractors
+ [ruutu] Add support for audio podcasts (#20473, #20545)
+ [xvideos] Extract all thumbnails (#20432)
+ [platzi] Add support for platzi.com (#20562)
* [dvtv] Fix extraction (#18514, #19174)
+ [vrv] Add basic support for individual movie links (#19229)
+ [bfi:player] Add support for player.bfi.org.uk (#19235)
* [hbo] Fix extraction and extract subtitles (#14629, #13709)
* [youtube] Extract srv[1-3] subtitle formats (#20566)
* [adultswim] Fix extraction (#18025)
* [teamcoco] Fix extraction and add suport for subdomains (#17099, #20339)
* [adn] Fix subtitle compatibility with ffmpeg
* [adn] Fix extraction and add support for positioning styles (#20549)
* [vk] Use unique video id (#17848)
* [newstube] Fix extraction
* [rtl2] Actualize extraction
+ [adobeconnect] Add support for adobeconnect.com (#20283)
+ [gaia] Add support for authentication (#14605)
+ [mediasite] Add support for dashed ids and named catalogs (#20531)

(leot)