Add phpcs in Composer

Summary:
We're going to add a phpcs check in Arcanist and have
already implemented it in the Jenkins tests job.

So, it makes sense to add it as a dev dependency in Composer.

Test Plan:
Ensure Composer is valid JSON.

jsonlint composer.json

Update composer dependencies, then check phpcs binary exists.

composer update
ls -lh vendor/bin/phpcs

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D139