Add a Composer linter

Summary: Adds a basic linter for ensuring that composer.lock files are up-to-date.

Test Plan: We have been using this in a private project for around a month.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: johnny-bit, Korvin

Differential Revision: https://secure.phabricator.com/D13883