fix script following new movers formula (bug 1059964)

fixes [bug 1059964](https://bugzilla.mozilla.org/show_bug.cgi?id=1059964)

This should fix the following cron error (previous fix was incomplete it seems):

```
Traceback (most recent call last):
  File "manage.py", line 93, in <module>
    execute_from_command_line(sys.argv)
  File "/data/mkt.prod/www/addons.mozilla.org/deploy-olympia-prod-20140912190542-8aa86cd45b/venv/lib/python2.6/site-packages/django/core/management/__init__.py", line 399, in execute_from_command_line
    utility.execute()
  File "/data/mkt.prod/www/addons.mozilla.org/deploy-olympia-prod-20140912190542-8aa86cd45b/venv/lib/python2.6/site-packages/django/core/management/__init__.py", line 392, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/data/mkt.prod/www/addons.mozilla.org/deploy-olympia-prod-20140912190542-8aa86cd45b/venv/lib/python2.6/site-packages/django/core/management/base.py", line 242, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/data/mkt.prod/www/addons.mozilla.org/deploy-olympia-prod-20140912190542-8aa86cd45b/venv/lib/python2.6/site-packages/django/core/management/base.py", line 285, in execute
    output = self.handle(*args, **options)
  File "/data/mkt.prod/www/addons.mozilla.org/deploy-olympia-prod-20140912190542-8aa86cd45b/olympia/apps/stats/management/commands/update_theme_popularity_movers.py", line 57, in handle
    persona_id=addon_to_persona[addon_id],
KeyError: 5L
```
Can we have something more explicit than `pop`?