Compatible with facebook HHVM 3.1

This patch to make HTML5 compatible with HHVM contains also workaround to avoid this https://github.com/facebook/hhvm/issues/2962
it is a phpunit/codecoverage limitation

I have commented it here https://github.com/goetas/html5-php/commit/f902b05cc252721e5440f3437d0c5de3e0a8d789#comments

It could blacklisted by default, but it is a @sebastianbergmann  issue