fix to duplicate column name: tz bug

Willie fails to load remind.py module due to repeat attempt to add column ['tz'] to preferences table in the setup function.

    Error in remind setup procedure: duplicate column name: tz
    (db.py:669)

Changing second add_column to ['time_format'] at line 56 fixes the bug and appears to be the intended behavior.
Merge pull request #405 from qubitsu/patch-1

[remind] fix to duplicate column name: tz bug