Explicitly use null default for timestamps in migrations.

I dropped in this while working on my app, thought I'd share. It's a small thing, but I hope this is helpful!

This clears the following deprecation warning in Rails 4.2.0.beta1, preparing for Rails 5:

==> Devise.orm = :active_record
DEPRECATION WARNING: `timestamp` was called without specifying an option for `null`. In Rails
5.0, this behavior will change to `null: false`. You should manually
specify `null: true` to prevent the behavior of your existing migrations
from changing.