installing 0.99.52 on php5.6.40

[ahnkle]

I was able to go through the install process, but when I tried to log in, I got an error:

Parse error: syntax error, unexpected '?' in /var/www/html/vfront/classes/src/User_Session.php on line 35

It seems the release assumes php7 (use of null coalescing operator, ??) but checks at start of install do not warn that php version will not work.