Add null => false to timestamp columns

Prior to Rails 4, timestamp columns were created as not nullable.  [Rails 4 reverted this behavior](https://github.com/rails/rails/commit/fcef728) 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.