Add null => false to timestamp column

Prior to Rails 4, timestamp columns were created as not nullable.  Rails
4 reverted this behavior to be not null be default. Kochiku started as a
Rails 3 app and migrated to Rails 4 so databases that were created after
the switch to Rails 4 do not have the not null constraint. This
migration adds it for all preexisting tables.
Merge pull request #66 from square/rob/not-null-timestamps

Add null => false to timestamp columns

R: rudle