Files can have defined minimal php version for linting

If the system's PHP version doesn't match the rule then the file is skipped.
The rule must be on the first line in the file, right after the opening php tag

```php
<?php // lint >= 5.4
```

This is especially usefull if you have traits in your project but support also 5.3.
You can [see the problem here](https://travis-ci.org/Kdyby/Doctrine/builds/26482051).

Check the travis build for example output.