Avoid errors if MULTIPLE_APP_SERVERS isn't set

As far as I can tell in the [settings docs](http://docs.readthedocs.org/en/latest/settings.html#multiple-app-servers), `MULTIPLE_APP_SERVERS` is not a required settings variable. However, if left undefined, the celery worker job will soft fail with a traceback that looks something like this:

```
AttributeError: 'Settings' object has no attribute 'MULTIPLE_APP_SERVERS'
readthedocs/projects/tasks.py:595 update_static_metadata
readthedocs/core/utils.py:78 copy_file_to_app_servers
django/conf/__init__.py:55 __getattr__
```