Declare PHPUnit as a Composer dependency

Summary:
IDE like PhpStorm are out of the box preconfigured to check
dependencies locally, so it's easier to

This also have the benefit to declare the license and to document
PHPUnit version.

Test Plan: run composer phpunit

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1818