Replaced custom container with a stub

The trick with custom container is no longer required with the latest phpspec versions. This PR makes calls to the container a bit more natural, since instead of calling `set()` we can now simply stub it:

```php
$this->container->get('router')->willReturn($router);
```

However, I'm not sure if the approach with container as a property is the road we want to continue going (see #3).