I18n - Allow custom translation loaders to be injected via module config #6244

This change allows modules to add their own translation loaders to the I18n translator via config like this:
```php
'translator_loaders' => array(
    'factories' => array(
        'my_new_loader' => 'ZendI18nDoctrineLoader\Factory\LoaderFactory',
    )
),
```
Before there was no way to add translation loaders without defining your own new translator service. See the ticket for more details: https://github.com/zendframework/zf2/issues/6244
In my original PR I removed the unused var $routePluginManager leaving L152 as:
$routePluginManagerFactory->createService($serviceLocator);

@Ocramius suggested that we keep the var and assert on it. 

After looking at this more, my personal opinion is that we are in the test for the TranslatorServiceFactory and should not be doing assertions against the RoutePluginManagerFactory's functionality.

If clean code is more important, I think the best thing to do is to remove this assertion and remove the unused $routePluginManager var. If PR relevance is more important, I could remove all changes to this test from this PR, leaving unused $routePluginManager var intact.