Fix storing cached values in Django 1.6

See: https://docs.djangoproject.com/en/dev/releases/1.6/#miscellaneous
There have been changes in the way timeouts are handled in cache
backends. Explicitly passing in timeout=None no longer results in using
the default timeout. It will now set a non-expiring timeout. Passing 0
into the memcache backend no longer uses the default timeout, and now
will set-and-expire-immediately the value.
Optimize has_translation(), it will use the cache sooner.
Add changelog entry for PR #34