Fix license in Composer

Summary:
README offers a BSD license, with a PURL to
https://opensource.org/licenses/BSD-2-Clause.

Composer used "BSD", but that's not a valid SPDX license tag.
Correct tag is BSD-2-Clause (the same than the OSI URL).

Reference: https://spdx.org/licenses/

Fixes T842.

Test Plan: composer validate

Reviewers: dereckson

Maniphest Tasks: T842

Differential Revision: http://devcentral.nasqueron.org/D406