fix an import loop in services (bug 1055654)

Stack trace of the issue (introduced in 986d8670de51cfd009bafec2b4aaa65335397860)

```
[magopian@dev1.addons.phx1 olympia]$ ../venv/bin/python services/wsgi/versioncheck.py
Traceback (most recent call last):
  File "services/wsgi/versioncheck.py", line 14, in <module>
    from update import application
  File "/data/addons-dev/www/addons-dev.allizom.org/deploy-olympia-dev-20140819092755-8ce8dd365c/olympia/services/update.py", line 25, in <module>
    from utils import (APP_GUIDS, get_mirror, log_configure, PLATFORMS,
  File "/data/addons-dev/www/addons-dev.allizom.org/deploy-olympia-dev-20140819092755-8ce8dd365c/olympia/services/utils.py", line 38, in <module>
    from amo.helpers import user_media_url
  File "/data/addons-dev/www/addons-dev.allizom.org/deploy-olympia-dev-20140819092755-8ce8dd365c/olympia/apps/amo/__init__.py", line 7, in <module>
    from caching.base import CachingQuerySet
  File "/data/addons-dev/www/addons-dev.allizom.org/deploy-olympia-dev-20140819092755-8ce8dd365c/venv/src/django-cache-machine/caching/base.py", line 10, in <module>
    from .compat import DEFAULT_TIMEOUT, FOREVER
  File "/data/addons-dev/www/addons-dev.allizom.org/deploy-olympia-dev-20140819092755-8ce8dd365c/venv/src/django-cache-machine/caching/compat.py", line 7, in <module>
    from django.core.cache.backends.base import DEFAULT_TIMEOUT as DJANGO_DEFAULT_TIMEOUT
  File "/data/addons-dev/www/addons-dev.allizom.org/deploy-olympia-dev-20140819092755-8ce8dd365c/venv/lib/python2.6/site-packages/django/core/cache/__init__.py", line 138, in <module>
    cache = get_cache(DEFAULT_CACHE_ALIAS)
  File "/data/addons-dev/www/addons-dev.allizom.org/deploy-olympia-dev-20140819092755-8ce8dd365c/venv/lib/python2.6/site-packages/django/core/cache/__init__.py", line 130, in get_cache
    "Could not find backend '%s': %s" % (backend, e))
django.core.cache.backends.base.InvalidCacheBackendError: Could not find backend 'caching.backends.memcached.MemcachedCache': Error importing module caching.backends.memcached: "cannot import name DEFAULT_TIMEOUT"
```
Might be interesting to put a comment on `amo.helpers.user_media_url` too? Only if we can't get rid of that one later.