youtube-dl: Update net/youtube-dl to 20180226

Changes:
version 2018.02.26

Extractors
* [udemy] Use custom User-Agent (#15571)

version 2018.02.25

Core
* [postprocessor/embedthumbnail] Skip embedding when there aren't any
  thumbnails (#12573)
* [extractor/common] Improve jwplayer subtitles extraction (#15695)

Extractors
+ [vidlii] Add support for vidlii.com (#14472, #14512, #14779)
+ [streamango] Capture and output error messages
* [streamango] Fix extraction (#14160, #14256)
+ [telequebec] Add support for emissions (#14649, #14655)
+ [telequebec:live] Add support for live streams (#15688)
+ [mailru:music] Add support for mail.ru/music (#15618)
* [aenetworks] Switch to akamai HLS formats (#15612)
* [ytsearch] Fix flat title extraction (#11260, #15681)

version 2018.02.22

Core
+ [utils] Fixup some common URL typos in sanitize_url (#15649)
* Respect --prefer-insecure while updating (#15497)

Extractors
* [vidio] Fix HLS URL extraction (#15675)
+ [nexx] Add support for arc.nexx.cloud URLs
* [nexx] Switch to arc API (#15652)
* [redtube] Fix duration extraction (#15659)
+ [sonyliv] Respect referrer (#15648)
+ [brightcove:new] Use referrer for formats' HTTP headers
+ [cbc] Add support for olympics.cbc.ca (#15535)
+ [fusion] Add support for fusion.tv (#15628)
* [npo] Improve quality metadata extraction
* [npo] Relax URL regular expression (#14987, #14994)
+ [npo] Capture and output error message
+ [pornhub] Add support for channels (#15613)
* [youtube] Handle shared URLs with generic extractor (#14303)

version 2018.02.11

Core
+ [YoutubeDL] Add support for filesize_approx in format selector (#15550)

Extractors
+ [francetv] Add support for live streams (#13689)
+ [francetv] Add support for zouzous.fr and ludo.fr (#10454, #13087, #13103,
  #15012)
* [francetv] Separate main extractor and rework others to delegate to it
* [francetv] Improve manifest URL signing (#15536)
+ [francetv] Sign m3u8 manifest URLs (#15565)
+ [veoh] Add support for embed URLs (#15561)
* [afreecatv] Fix extraction (#15556)
* [periscope] Use accessVideoPublic endpoint (#15554)
* [discovery] Fix auth request (#15542)
+ [6play] Extract subtitles (#15541)
* [newgrounds] Fix metadata extraction (#15531)
+ [nbc] Add support for stream.nbcolympics.com (#10295)
* [dvtv] Fix live streams extraction (#15442)

(leot)