Upgrade the Makefile-specified pip version to 8.1.2
Upgrade the Makefile-specified pip version to 8.1.2

Summary:
8.1.2 is latest, and what we get by default on install.
This moves to that, and removes use of deprecated flags.

Test Plan: Manual/Changes

Reviewers: paulruan

Reviewed By: paulruan

Subscribers: changesbot

Differential Revision: https://tails.corp.dropbox.com/D201353

dropbox/changes,dropbox/changes,dropbox/changes,dropbox/changes