Fixed class autoloading

We should use PSR-4 because `target-dir` field is now deprecated on Composer.

From the Composer doc:

> **DEPRECATED**: This is only present to support legacy PSR-0 style autoloading,
> and all new code should preferably use PSR-4 without target-dir and projects
> using PSR-0 with PHP namespaces are encouraged to migrate to PSR-4 instead.