Require Laravel 5.2, but not Laravel 5.3

Summary:
The Docker Image doesn't build, as at artisan optimize step,
a fatal error occurs, as our EventServiceProvider::boot() isn't
compatible anymore with parent class signature.

This is caused by two factors:

• upstream: commit fa9f3220 made the class boot method now use facades instead of dependency injection to the method
• locally: commit d69820ed127c required Laravel 5.2.41+ to get session code update, but didn't guard agaisnt 5.3.*

We require 5.2.*, using ~ and not ^ in Composer version syntax.
The caret operator is more appropraited when semver is better followed.

References:

• https://getcomposer.org/doc/articles/versions.md#next-significant-release-operators
• https://getcomposer.org/doc/faqs/why-are-unbound-version-constraints-a-bad-idea.md

This is a follow-up for d69820ed127c.
Fixes T997.

Test Plan: Ensure composer install latest 5.2.

Reviewers: Sandlayth

Maniphest Tasks: T997

Differential Revision: https://devcentral.nasqueron.org/D638