Update install_ubuntu.sh to the new age

Summary:
Ref T4200. Since the last time this script was written, Ubuntu has made lots of changes.
Try to keep up with those.

Test Plan:
Ran this on frash installs of Ubuntu 16.04 LTS and 18.04 LTS (Pre-release).
Got to see Phabricator running.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: amckinley, Korvin

Maniphest Tasks: T4200

Differential Revision: https://secure.phabricator.com/D19394