Limit django-extra-views and haystack range

* django-extra-views 0.6.5 added a pinned version of six, which causes
  issues with other dependencies that install a different version. A fix
  is already on master, but it's easier to limit the range for now.
* Haystack 2.2.0 was released and looks safe to use with Oscar. I'll
  allow that for now, but in general we should probably limit the amount
  of versions we support.