Added exclude to UnitChangeForm.meta

Hi Chrisspen, 

I added an exclude field to UnitChangeForm.meta b/c I was getting a error when building the database:
"django.core.exceptions.ImproperlyConfigured: Creating a ModelForm without either the 'fields' attribute or the 'exclude' attribute is prohibited; form UnitChangeForm needs updating."

Its a tiny change but I wanted to figure out how to contribute to public projects on github and this is a project I'm interested in.