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

Be sure to also read my point about why this is better: https://github.com/doctrine/doctrine2/pull/1056#discussion_r13665544
We don't want this. We want to switch to ``newInstanceWithoutConstructor`` only for versions affected by the unserialization BC break, because switching to newInstanceWithoutConstructor for any 5.4+ version introduces other issues