Update index.php for slimapp migration

Summary:
Use \Wikimedia\Slimapp\Config instead of the deleted
\Wikimedia\IEGReview\Config class to load configuration from disk.

Test Plan: Apply patch, see the app load rather than crash

Reviewers: Niharika, bd808

Reviewed By: bd808

Differential Revision: https://phabricator.wikimedia.org/D1129