ensure that all libs from composer are compatible with php 7.2 and enforce 7.2...

ensure that all libs from composer are compatible with php 7.2 and enforce 7.2 by settings config.platform.php to 7.2, fixes #1569

Closes #1569

Merge request studip/studip!1002

composer.json

@@ -2,7 +2,10 @@
     "name": "studip/core",
     "description": "Stud.IP ist ein Campus- und Lernmanagementsystem für Hochschulen, Schulen, Unternehmen, Verbände und Behörden.",
     "config": {
-        "vendor-dir": "composer"
+        "vendor-dir": "composer",
+        "platform": {
+            "php": "7.2"
+        }
     },
     "require-dev": {
         "adlawson/vfs": "~0.12.1",