Upgrade PHP to 5.6.18
Upgrade PHP to 5.6.18

Summary: PHP 5.6.18 contains security fixes.

Test Plan: `docker build`

Reviewers: dereckson

Subscribers: #nasqueron_docker_deployment_squad, Kaliiixx

Projects: #docker_images

Differential Revision: http://devcentral.nasqueron.org/D270

nasqueron/docker-nginx-php-fpm,nasqueron/docker-nginx-php-fpm