Fix an error when getting results from DB in get_student_groups.

The error affected the Python 2.6 version we distribute for
Windows. Modern versions of Python, and even 2.6 on Windows,
are not affected.

The key 'group_id' was not in the variable row. 'StudentGroups.group_id'
needed to be used instead.
Update version number and changelog.
Merge branch 'hotfix-0.4.3'.