[WIP] Fix instantiation for latest PHP versions.

PHP 5.4.29 and 5.5.13 introduced a BC break that prevents using serialization as a means for instantiating internal objects without calling their constructor (unless they implement `Serializable`) [[1]]. This issue is still evolving and workarounds are currently being discussed in the PHP internals mailing list [[2]]. 

In the mean time, since many PHP libraries rely on this functionality which is becoming increasingly complicated to support, @Ocramius has created [`ocramius/instantiator`](https://github.com/Ocramius/Instantiator) [[3]]. This PR adds support for `ocramius/instantiator` so that phpspec can continue to support this feature and stay in sync with other PHP libraries which support this feature without having to duplicate logic/updates across multiple projects.

Please note that switching to `ocramius/instantiator` doesn't immediately solve all problems related to this BC break. Right now some of our tests are still failing. Ideally, as workarounds become available, they will be added to `ocramius/instantiator`.

[1]:http://news.php.net/php.internals/74654
[2]:http://marc.info/?t=140347878100001&r=1&w=2
[3]:https://twitter.com/s_bergmann/status/475284594141691904
The instantiator already caches internally, so it's pretty ok.