Install APCu from stable PEAR channel

Summary:
PHP 5.6 requires the APCu stable version.

Currently, the beta version offers an APCu for PHP7+.

Fixes T605.

Test Plan: Task description includes the operation log.

Reviewers: Kaliiixx

Subscribers: Nasqueron Docker deployment squad

Projects: Docker images

Maniphest Tasks: T605

Differential Revision: http://devcentral.nasqueron.org/D96