Require PHP 7.0 during dependency resolution, even if running 7.1+ (#106)

Author: greg-1-anderson

composer.json

@@ -103,6 +103,9 @@
   "config": {
     "optimize-autoloader": true,
     "preferred-install": "dist",
-    "sort-packages": true
+    "sort-packages": true,
+    "platform": {
+      "php": "7.0"
+    }
   }
 }