Fix filtering by region in feed elements with fireplace (bug 1061206)

https://bugzilla.mozilla.org/show_bug.cgi?id=1061206
Fix for http://sentry.dmz.phx1.mozilla.com/marketplace-dev/marketplace-dev/group/20970/

```
mkt.feed.views in _do_app_filter
KeyError: 'regions'

  File "rest_framework/views.py", line 396, in dispatch
    response = handler(request, *args, **kwargs)
  File "mkt/feed/views.py", line 788, in get
    data = (self.filter_apps_feed_element(request, dict(data)) or
  File "mkt/feed/views.py", line 485, in filter_apps_feed_element
    map_field='slug')
  File "mkt/feed/views.py", line 533, in _filter
    app for app in feed_element['apps'] if _do_app_filter(app)]
  File "mkt/feed/views.py", line 518, in _do_app_filter
    app[field] = map(lambda x: x[map_field], app[field])
```
We don't need a non-feed discoplace serializer for now.