put a minimal config for installer into config dir

config/installer.php

@@ -0,0 +1,10 @@
+<?php
+
+return [
+    'php' => [
+        # You can change this to a lower version, the lowest
+        # can be 7.0. However, there's no guarantee that things
+        # will work properly. Change at your peril!
+        'version' => '7.1.3'
+    ]
+];