Upgrade to PHP 7.1

Summary:
As the notifications center now uses PHP 7.1,
we so switch to the relevant PHP 7 base image.

Fixes composer issue as it tries to validate dependencies
requirements for dev, even when --no-dev is passed.

Ref T1062.

Test Plan:
Rebuild the container: docker build -t nasqueron/notifications .

Run tests.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1062

Differential Revision: https://devcentral.nasqueron.org/D782