Autocomplete filter doesn't work with new version of Grappelli

[cinevillenl]

Autocomplete AJAX calls will fail due to the to_field parameter being renamed.

See this commit specifically: https://github.com/sehmaschine/django-grappelli/commit/a1cbbda3ec51f48672ec93fc6da3937c8d71691c#diff-7395aa4bd4df81d0cb6453995851765b