Added HHVM and PHP 5.6 to travis-ci

run build against PHP 5.6 and hhvm but allow them to fail to just see how it works.

http://docs.travis-ci.com/user/build-configuration/#Rows-That-are-Allowed-To-Fail
you have an invalid YAML here ``[`` is the marker for a YAML array, which is invalid in this context. You need to quote the YAML string