Add `null: true` to avoid DEPRECATION WARNING

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
Merge pull request #489 from yahonda/rails42_timestamp_null_true_explicitly

Add `null: true` to avoid DEPRECATION WARNING