Supersede raven/raven dependency by sentry/sentry

Summary:
The package raven/raven is deprecated, and moved to another location
on Packagist.

Fixes T871.

Test Plan:

• In php artisan tinker: App::make('raven')
• Run SentryServiceProviderTest unit test

Reviewers: dereckson

Subscribers: alken-orin

Maniphest Tasks: T871

Differential Revision: https://devcentral.nasqueron.org/D533