Use newInstanceWithoutConstructor when available.

Changed ClassMetadataInfo::newInstance to always use ReflectionClass::newInstanceWithoutConstructor when available, not just for specific versions. 

As discussed here: https://github.com/doctrine/doctrine2/commit/530c01b5e3ed7345cde564bd511794ac72f49b65#commitcomment-6635975
I'm curious, why is this better than  `PHP_VERSION_ID >= 50400` (which I submitted over an year ago in #588)? They will reject it anyway, see also #1053 and #1050.