allow more versions of the ACL package

This actually is the version that is required when using the Symfony SE
(`symfony/symfony` requires `symfony/security-acl` in version `~2.7`).

src/Symfony/Bundle/SecurityBundle/composer.json

@@ -18,7 +18,7 @@
     "require": {
         "php": ">=5.3.9",
         "symfony/security": "~2.8|~3.0.0",
-        "symfony/security-acl": "~2.8|~3.0.0",
+        "symfony/security-acl": "~2.7|~3.0.0",
         "symfony/http-kernel": "~2.2|~3.0.0"
     },
     "require-dev": {