Fix BC break with skeleton for Translator Service

- Mvc\TranslatorServiceFactory was missing the "implements FactoryInterface"
 declaration, making it fail as a valid factory.
- Circular dependency was occurring, as changes in TranslatorServiceFactory
 meant it was looking for existence of an alias already defined in the
 skeleton, and pointing back at the same service. Added a
 'Zend\I18n\Translator\TranslatorInterface' service, pointing to the I18n
 TranslatorServiceFactory, and now check for that in the MVC
 TranslatorServiceFactory. This fixes the primary BC issue.
- Updated View\HelperPluginManager to look for (and conditionally fetch) the
new
 'Zend\I18n\Translator\TranslatorInterface' service, instead of the
 'translator' service. This will preserve the intent. Developers may alias
 'translator' to 'Zend\I18n\Translator\TranslatorInterface' if desired.