adjust required Propel minimum version

Author: havvg

composer.json

@@ -17,7 +17,8 @@
   "require": {
     "symfony/symfony": "^2.8.2|^3.0.1",
     "symfony/propel1-bridge": "2.7.x-dev",
-    "propel/propel1": "~1.6"
+    "propel/propel1": "^1.6.8",
+    "symfony/security-acl": "@stable"
   },
   "require-dev": {
     "sensio/framework-extra-bundle": "^3.0.2",