fixing "global name DuplicateKeyError is not defined" error

I was running some tests and ran into the following error:

```
  File ".../mongomock/mongomock/collection.py", line 59, in _insert
    raise DuplicateKeyError("Duplicate Key Error", 11000)
NameError: global name 'DuplicateKeyError' is not defined
```

This fixes it.