Add suggested phpunit/php-invoker to our global composer config

It's unclear if this will ever actually be a requirement for us, but it's not a horrible thing to have.

See https://packagist.org/packages/phpunit/php-invoker for full details. Invokes callables with a timeouts.
Merge pull request #242 from 10up/250-invoker

Add suggested phpunit/php-invoker to global composer config