Change the default `null` value for timestamps

As per discussion, this changes the model generators to specify
`null: false` for timestamp columns. A warning is now emitted if
`timestamps` is called without a `null` option specified, so we can
safely change the behavior when no option is specified in Rails 5.
Merge pull request #16481 from sgrif/sg-change-default-timestamps

Change the default `null` value for timestamps