Fix UnicodeEncodeError in ProductVariation.__unicode__

field.verbose_name.decode(utf-8) raises UnicodeError if verbose_name can
not be encoded by ASCII.

This error occurs when you apply a translation to field.verbose_name and try to access the product admin. As the translation is (usually) not encodable by ASCII, a UnicodeEncodeError ('ascii' codec can't encode characters in position 0-1: ordinal not in range(128)) is raised.