composer.{json,lock} - Make the "tplaner/when" exception for old PHP reproducible

composer.{json,lock} - Make the "tplaner/when" exception for old PHP reproducible

Overview
--------

The library `tplaner/when` has an inaccurate dependency on php71 and
requires special work to deploy on php70.

Before
------

The `composer.lock` file was manually edited to make it runnable on php70.
However, if you `rm composer.lock && composer install`, then it fails.

After
-----

The `composer.json` and `composer.lock` are more repeatable.  If you run
`rm composer.lock && composer install`, then it works.

Comments
--------

This adds a fairly ugly section.  However, the good news is that it should
be more transparent - which should make it easier to understand/remove
later.

As a general rule, we don't want to have `civicrm-core.git` depend on
packages/versions that aren't published.  This is a somewhat unusual
edge-case - this package/version is published via `packagist.org`.  If you
run without this on php71+, then you still get a valid/equivalent package.
All that we've done is to enable running on `php70` (when `civicrm-core` is
a root project).