Install youtube-dl on viperserv role

Summary:
Required by e6a81e6f29f4.

Ref T1309

Test Plan: Give a YouTube URL on a channel where Dæghrefn is connected.

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: ledesillusionniste

Maniphest Tasks: T1309

Differential Revision: https://devcentral.nasqueron.org/D1229