change import style

@twymer this was giving me the following error when I tried to do anything in the django shell:

```
  File "/usr/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/home/skelly/dev/projects/commcare-hq/corehq/apps/importer/tasks.py", line 9, in <module>
    import corehq.apps.importer.util as importer_util
AttributeError: 'module' object has no attribute 'apps'
```