Use Django 1.7 StaticLiveServerTestCase when available

Lettuce DjangoServer uses LiveServerTestCase to implement his own server.

Django 1.7 changed the way LiveServerTestCase works and introduced the StaticLiveServerTestCase, to be used to hit static files in different places without run collectstatic:

https://docs.djangoproject.com/en/1.7/howto/static-files/#testing

Without it, it would be necessary to call collectstatic before run tests. I suggest to try import StaticLiveServerTestCase before LiveServerTestCase to match django 1.7.