minor #15651 Require actual version of ACL component until ACL/2.8 br…

…anch released (scaytrase)

This PR was squashed before being merged into the 2.8 branch (closes #15651).

Discussion
----------

Require actual version of ACL component until ACL/2.8 branch released

symfony/2.8@stable is not released, so version reference is incorrect.

This will allow require 2.8.x-dev version alongside with symfony/symfony:2.8.x-dev

Fix #15648

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | BC fix
| Deprecations? | no
| Tests pass?   | [not sure](https://travis-ci.org/scaytrase/symfony/builds/77840665)
| Fixed tickets | 15648
| License       | MIT
| Doc PR        | none

Commits
-------

2c7caaa Require actual version of ACL component until ACL/2.8 branch released

composer.json

@@ -20,7 +20,7 @@
         "doctrine/common": "~2.3",
         "twig/twig": "~1.20|~2.0",
         "psr/log": "~1.0",
-        "symfony/security-acl": "~2.8"
+        "symfony/security-acl": "~2.7"
     },
     "replace": {
         "symfony/asset": "self.version",