Hhvm testing

For testing hhvm on travis, and for aiming to add ezpublish-kernel to [hhvm framework tests](https://github.com/facebook/hhvm/blob/master/hphp/test/frameworks/frameworks.yaml), but we need to have this in master to show we are dedicated to try to add support before we should do that.

Note: This will fail (fatal on hhvm 2.x and run but fail on 3.x), which is why hhvm is added to "allowed failure".

Todo:
 - [x] Get code running on HHVM (with 3.0.0-dev for XSLT support)
 - [x] Fix issues in code/tests detected by hhvm (but not issues which are clear HHVM issues)
 - [x] Wait for travis to deploy HHVM 3.0.x, then cleanup and ping for review