Upgrade code to PHP 7.1

Summary:
The current code works virtually without issue on PHP 7,
as it was prepared with PHP 7 in mind as target version.

In order to work nicely with PHP 7.1:

• use nullable parameters type
• fix ProjectsMapTest tests mock type, using an anonymous class

We also bump the version in Composer.

References:

• https://wiki.php.net/rfc/nullable_types
• http://php.net/manual/en/language.oop5.anonymous.php

Fixes T1064. Ref T1062.

Test Plan: All unit tests pass.

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: alken-orin

Tags: PHP 7 deployment

Maniphest Tasks: T1064, T1062

Differential Revision: https://devcentral.nasqueron.org/D753