Allow the PHP5.5-build to fail

PHP5.5 is not officially released, so making it part of
the overall build-success seems not useful. It can be
simply re-enabled it, when PHP5.5 is released, or as
soon as the builds doesn't fail anymore.
Merge pull request #1473 from KingCrunch/allow-5.5-fail

Allow the PHP5.5-build to fail