[TASK] Add hhvm to travis

Running the test suite with hhvm shows some code smells and
actual issues the core with this interpreter. While unit tests
are in a fairly good shape already, functionals are still
totally broken.

Having hhvm in our standard test suite in travis as
"allowed failure" is a first step to raise awareness towards
this topic.

Change-Id: Ia684965af9e43ccbce791a10ba664fc49a86191a
Resolves: #62783
Releases: master
Reviewed-on: http://review.typo3.org/33902
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>