Add more PHP versions to Travis build

- Run in 5.6 version
- Run in hhvm
- Run in hhvm-nightly

It allows HHVM to fail because PHPUnit [does](https://github.com/sebastianbergmann/phpunit/blob/master/.travis.yml#L12-L15) and since you're using PHPUnit to validate the build, there is no reason for do in a different way, IMO.