Full locale middleware path in installation.rst

Just putting `UserenaLocaleMiddleware` in `settings.py` results in an error:

    ImproperlyConfigured: UserenaLocaleMiddleware isn't a middleware module

I probably won't be the last person to just copy-and-paste from `installation.rst` and have to spend a few moments figuring out what to do (the [UserenaLocaleMiddleware docs](http://docs.django-userena.org/en/latest/api/middleware.html) are unfortunately blank to the extent that the actual import path isn't shown).