title: Make column not nullable in a Laravel migration
body: <p>I'm writing a migration to make certain columns in a table <code>nullable</code> right now. For the down function, I of course want to make those columns <code>not nullable</code> again. I looked through the <a href="http://laravel.com/docs/database/schema" rel="noreferrer">schema builder docs</a>, but couldn't see a way to do this. </p>
  
  <p>Any help would be appreciated.</p>