Fixed case sensitive comparision

This line is causing [the Travis CI failures](https://travis-ci.org/mozilla/kuma/builds/33392632). [`filter` is a django method where keyword arguments match field lookups.](https://docs.djangoproject.com/en/1.4/ref/models/querysets/#django.db.models.query.QuerySet.filter) Django provides [`iexact`](https://docs.djangoproject.com/en/1.4/ref/models/querysets/#iexact) to match on a case-insensitive value, so this should be `filter(name__iexact=tag)`.