Replaced phpcs --version by PHP_CodeSniffer::VERSION in version check to improve Composer compatibility.

Composer users are not able to use `PhpCodeSnifferTask` without `skipversioncheck="true"` because `phpcs` is not on system's path by default, so `version_compare` always fails. However, we can use Composer-agnostic `PHP_CodeSniffer::VERSION` const, so that's what I did.