HipHop VM testing

[Travis CI added hhvm](http://about.travis-ci.org/blog/2013-12-16-test-php-code-with-the-hiphop-vm/) as possible PHP version to test against. Because hhvm is an emerging alternative PHP VM it's interesting to keep an eye on it. By adding it to the test matrix we can see how our code (changes) respond on hhvm. Since it's not critical and AFAIK a goal to work on hhvm I've added it to allow failures. So we don't have to worry about breaking the tests if it doesn't work in hhvm just yet.