Fix: Phix overwrites 'project.stability' from 'package.xml'

Phix should not override the 'project.stability' from 'package.xml' unless we are building a snapshot release or the property is not set / empty.

Otherwise, it would be impossible to adhere to the package stability standards defined by PEAR as Phix would not allow us to choose anything besides 'stable' or 'snapshot'.

**See:** http://pear.php.net/manual/de/guide.users.concepts.stability.php