drop HHVM and PHP5.x support

HHVM is used nearly anywhere and since php7 is a thing also doesnt has a real use-case anymore.

it was a great project to boost zend-php though.

we should drop hhvm and php5.x support and cleanup the code-base from hhvm workarounds (and drop hhvm ci).

do you guys agree? If so, I can provide a PR.