Prepare master for eZ Publish 5.4.0-alpha

With these changes master becomes 5.4.0-alpha, and with it the minimum PHP requirement is increased to PHP 5.4.4 (the version used by Debian 7). In addition this PR adds work in progress support for HHVM, and same for PHP 5.6

Todo:
- [ ] Bump StashBundle to v0.4 (support HHVM)
- [ ] Optional: Go over other dependencies and attempt to use newer versions if available.

Note1: This PR replaces earlier PR to add HHVM(#735).
Note2: Next expected PHP version requirement bump (to PHP 5.5) is next year when Debian 8 [is expected](https://lists.debian.org/debian-devel-announce/2013/12/msg00008.html) to come out in stable release.