Phpunit 6 compatibility

[soullivaneuh]

This one is a quite easy-pick and can be release under a new minor because of the PHP version dropping.

[soullivaneuh]

@stof Failing HHVM seems to be not related.

The last master build is form over 2 years now, maybe you can try a new trigger?

[gustawdaniel]

I propose to remove HHVM support, because in my opinion support for phpunit 6 is most important.

[kevin-lot]

@Soullivaneuh I did a pull request huge upgrades. Use Phpunit 6.5 and remove HHVM are parts of it.