Fix for ClassMetadata::newInstance() PHP 5.5.13 

Fixes https://github.com/doctrine/phpcr-odm/issues/498
Fix for ClassMetadata::newInstance() PHP 5.5.13

Improved fix for https://github.com/doctrine/phpcr-odm/issues/498

Do it the same way as Doctrine ORM does. The newInstanceWithoutConstructor() is available for PHP 5.4 and higher only, so a check against the used PHP version is required.
Merge pull request #499 from joschi127/master

Fix for ClassMetadata::newInstance() PHP 5.5.13