Merge pull request #18 from tsantos84/master

Added symfony Yaml 4 to project dependencies

Dropping HHVM support

Fixes

Update composer.json

Serialization of SplFixedArray and classes extending from it

Fixing SplFixedArray not being serializable

Changed object cache from protected to protected static

Added backslash to PHP internal functions

Update DeepCopySerializer.php

DeepCopySerializer