Firstly... that ACL was designed for Laravel 4. Laravel 4.2 actually has RBAC built into it so you don't need to use that.
There are multiple changes between versions 4, 4.1 and 4.2. So even if you did install it with composer it wouldn't work.
Secondly... Laravel 4.2 is still in development so I wouldn't use it yet. Composer is reporting that you need to replace laravel 4.2.* with 4.0.*.
You could update the composer file for it to require laravel: 4.2.* but I'd advise against it (you'd just be coming back here as it will provide more issues)
---Edit--- Ignore the bit about RBAC being built into 4.2... I read that in January from one of the framework developers... alas I haven't found it after going through it. However there's plenty of guides for rbac and acl. Just don't copy and paste.