Fix PHP Strict mode compatibility

Running evaluate PHP 5.4.13 in strict mode results in the following notice:

```
PHP Strict standards:  Non-static method Netresearch\Logger::printOnOutput() should not be called statically in
/Volumes/Work/workspace-ssd/judge/lib/Netresearch/Logger.php on line 61
PHP Stack trace:
PHP   1. {main}() /Volumes/Work/workspace-ssd/judge/judge:0
PHP   2. Symfony\Component\Console\Application->run() /Volumes/Work/workspace-ssd/judge/judge:28
PHP   3. Symfony\Component\Console\Application->doRun() /Volumes/Work/workspace-ssd/judge/vendor/symfony/console/Symfony/Component/Console/Application.php:118
PHP   4. Symfony\Component\Console\Command\Command->run() /Volumes/Work/workspace-ssd/judge/vendor/symfony/console/Symfony/Component/Console/Application.php:194
PHP   5. Judge\Evaluate->execute() /Volumes/Work/workspace-ssd/judge/vendor/symfony/console/Symfony/Component/Console/Command/Command.php:226
PHP   6. Netresearch\Logger::printResults() /Volumes/Work/workspace-ssd/judge/src/Judge/Evaluate.php:131
```
Merge pull request #10 from Vinai/master

merge #10: Fix PHP Strict mode compatibility