Investigate issues in HHVM

[Gisleburt]

Using the Docker container hhvm/hhvm and the Travis-CI build create different problems. See #16

[JonathanO]

If the __destruct on refcount hitting 0 behaviour is no longer a requirement for an implementation of the PHP language then this kind of code is going to have to be rewritten to be more explicit about cleanup. If this really is the way the language is going then __destruct will be about as useful as a finalizer in Java, and explicit close() methods are going to be needed in various places.