Running into the same issue...
HHVM :8000 3.11.1+dfsg-1ubuntu1/ 5.6.99-hhvm
running in fcgi mode, with fallback to
php7-fpm running at local socket.
After stopping the hhvm service, the install routine did work in my case...
So this is related to hhvm.
i replaced hhvm with php7 and everything worked fine. The installer check needs to warn if hhvm is not supported, not leave us to scratch our heads to find out
Have you tried to disable hhvm:
sudo service hhvm stop sudo service php5-fpm start