Rename Weak to Ref and upgrade to work with latest php-ref ext

While this release is a patch-release, it may introduce some BC-incompatible changes because of improvements and bug fixes in php-ref extension.

As of php-ref v0.2.4 notifiers get called even in case of exception thrown in previous notifiers or referent object destructor, which means that WeakMap now will be properly cleaned up in such cases.