Add APCu extension

Summary:
Our downstreams containers use frequently APCu.

The version 4.0.8, released the 2015-11-20, is considered stable.

We so incorporate the extension directly to our base image.

Test Plan:
Build the image, run a container, check phpinfo().

php -i relevant results are available at P137.

Reviewers: Kaliiixx

Subscribers: Nasqueron Docker deployment squad

Projects: Docker images

Maniphest Tasks: T604

Differential Revision: http://devcentral.nasqueron.org/D97