Maintenance for base library usage

[EmanueleMinotto]

Just a quick touch of maintenance for:

1. Travis: the trusty distribution is now required to install PHP <= 5.5, see https://travis-ci.community/t/php-5-4-and-5-5-archives-missing/3723
2. PHPUnit: from PHP 7.2 the usage of PHPUnit v4 will give warnings, PHPUnit v5 is still compatible with current tests and won't give messages
3. exclude tests from production autoload, those shouldn't be available at all
4. bump for current graph version

I hope you will appreciate ✌️

[EmanueleMinotto]

ping @SammyK