Do not provide default value for Change.translation field

This value is never used and causes problems with some
MySQL/Django/South version combinations.

The only reason this field originally had NULL attribute and it has it
now removed was migration path, where it was filled in by separate data
migration:

0018 - adds Change.translation field with NULL allowed and default
0019 - fills in Change.translation for all entries
0020 - removes NULL attribute

This fixes #319.