NEMO-1838 Add :null => false to timestamps.

In Rails 3.2, :null => false was added to migration timestamps by default.
In Rails 4.0, that change was reverted.

Since our migrations were run in Rails 3.2, our DB schemas
(including production) have :null => false.

This is a problem when a developer does

    rake db:drop
    rake db:create
    rake db:migrate

Because db/schema.rb will be different than what is checked
in to github.

To fix this without forcing migrations to be run on all
merchant databases in production, I am adding :null => false
to the EXISTING migrations. So if they get re-run, they do
the same thing they used to do.

This DOES NOT mean we have to add :null => false to NEW
migrations starting in Rails 4.0. We don't!