Update youtube-dl to version 2013.11.15 to fix subtitle downloading.

This version contains a fix for an issue which prevents the old version from downloading subtitles:

https://github.com/rg3/youtube-dl/issues/1750

Previously, I would get 404's when attempting to download subtitles with edu_10gen_dl.

I can confirm that with this upstream fix in youtube-dl, subtitle downloading now works.