Run travis tests on HipHop Virtual Machine (hhvm).
The HHVM environment does not know phpize, so we can not install extensions.
Merge pull request #211 from bantu/travis/hhvm

Run travis tests on HipHop Virtual Machine (hhvm).

* bantu/travis/hhvm:
  The HHVM environment does not know phpize, so we can not install extensions.
  Run travis tests on HipHop Virtual Machine (hhvm).