PHP 7.2 compatibility vol.2

[xificurk]

Regarding PHP 7.2 compatibility, it turns out #9 is not enough. Although the existing tests passed, you run into a problem when you actually execute the code of DoNotExtendNetteObjectRule or NetteObjectClassReflectionExtension, because $reflection->isSubclassOf() triggers the autoloading and it dies on PHP 7.2. This PR solves the problem by:

• Adding tests for both classes, so the code is actually executed during travis build.
• Rewriting the code so that it avoids triggering autoloading of Nette\Object.
• There is a small BC break - DoNotExtendNetteObjectRule now checks only the direct parent of a class. But I consider this a bugfix - PHPStan should report problems with project code only, not problems with classes from 3rd party libs that are used and extended in the project.

[ondrejmirtes]

Thanks a lot, awesome PR! :)

BTW: I think that PHPStan triggers autoloading of classes in more cases than it should, I'll try to address this.