create_purchasers: Remove `DEFAULT UTC_TIMESTAMP()`

Ended up with this error when running my migrations against the
production server:

    error: migration failed in line 0: BEGIN;

    CREATE TABLE purchasers (
        id INT AUTO_INCREMENT PRIMARY KEY,
        name VARCHAR(255) NOT NULL,
        email VARCHAR(255) NOT NULL,
        secret VARCHAR(255),
        created_at DATETIME NOT NULL DEFAULT UTC_TIMESTAMP(),
        updated_at DATETIME NOT NULL DEFAULT UTC_TIMESTAMP()
    );

    CREATE TRIGGER purchasers_updated_at
        BEFORE UPDATE
        ON purchasers FOR EACH ROW
            SET NEW.updated_at = UTC_TIMESTAMP();

    COMMIT;
     (details: Error 1064: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'UTC_TIMESTAMP(),
        updated_at DATETIME NOT NULL DEFAULT UTC_TIMESTAMP()
    );

    CR' at line 6)

Using MySQL 5.6.34 in production and MariaDB 10.3.10 locally. Guess you
can't use `DEFAULT UTC_TIMESTAMP()` there.

To use `UTC_TIMESTAMP()`, create a couple new triggers to set the
timestamp in the `created_at` and `updated_at` columns.

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