Use PHP 7.1 Docker image

Summary:
As a part of our PHP 7.1 deployment, we switch CI to this version too.

We follow the PHPUnit recommendation about versionning and ship PHPUnit 5.7.
Reference: https://github.com/sebastianbergmann/phpunit/wiki/Preparing-for-PHPUnit-6

Futhermore, locales is now provided by base image.

Ref T1073. Fixes T1068.

Test Plan: Build the image and run an Ant build for rNOTIF

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1073, T1068

Differential Revision: https://devcentral.nasqueron.org/D766