oldphp Branch #28
Description
After some research, it seems that many projects are sticking on older versions of the original library to support PHP versions below 7.1 (e.g. Codeception/module-rest#24)
This is fine at some point because we also provide branches for PHP 7.0 and 5.6. However, there's also a need for PHP 5.5 and 5.4, but creating separate branches for these versions isn't the best idea when it comes to maintainability.
For this reason, the branches php-56 and php-70 should be replaced with a single Branch named oldphp, which supports all older versions (5.4 - 7.0) at once. The only problem is PHPUnit < 6, which doesn't support PSR-4 namespaces, thus we may have to create shims, which provide us access to all required PHPUnit related classes, regardless of its version. Alternatively, just don't do tests, which would allow us to lower the PHP requirements, at least to 5.3, because we wouldn't need support for traits.