Drop unused features and fix bugs

NOTE: this release drops PHP 7.0 support!. Since php-ref 0.5.0 PHP >= 7.1 required!

This release fix bugs, drop unused feature and soon to be EOL PHP 7.0 (see http://php.net/supported-versions.php for details).

Changelist:

• Drop PHP < 7.1 support (BC-breaking);
• Remove array notifier in a favor of callback (BC-breaking);
• Remove references comparison (BC-breaking);
• Fix bug when object handle reused but previous state was not clean;
• Fix wrong behavior when reference get deleted during notifier call;
• Fix memleaks;