Fix non-explicit dependencies

Getting this error today
```
Problem 1
    - zendframework/zend-cache 2.3.x-dev requires zendframework/zend-eventmanager == 2.3.9999999.9999999-dev -> no matching package found.
    - zendframework/zend-cache 2.3.x-dev requires zendframework/zend-eventmanager == 2.3.9999999.9999999-dev -> no matching package found.
    - Installation request for zendframework/zend-cache 2.3.x-dev -> satisfiable by zendframework/zend-cache[2.3.x-dev]
```

Previous `composer.json` will fail (started from 2014-08-12 11:58 UTC) on anyone project that require this library with minimum stability `dev` since `zendframework/zend-eventmanager` doesn't have `2.3-dev` branch.