title: Removing default value from Rails Migration
body: <p>I found several similar questions about editing a migration but couldn't figure this one out. I did a rails migration, then opened the migration file and added a default value option to the field. Then ran rake db:migrate. The default value populates as intended. Then a few migrations later, I decided that I wanted to remove the default value option. How do I do that? </p>
  
  <p>If this was the last migration I did, I would use db:rollback and recreate but since was done a few migrations ago, I'm not sure how to fix this. </p>
  
  <p>Appreciate the help.</p>