Install unrar on shell servers

Summary:
If tar and xz are installed by default, RAR support isn't.

Note this software isn't open source, but only freeware.
On Debian, it requires nonfree repository.

Test Plan: unrar x archive.rar

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1155