title: Rails 4: Remove not null constraint from table column with migration?
body: <p>Given the following <code>schema.rb</code>:</p>
  
  <pre><code>  create_table "people", force: true do |t|
      t.string   "name",  null: false
      t.integer  "age"
      t.integer  "height"
      t.string   "email"
      t.boolean  "married",  default: false
      t.text     "bio"
      t.integer  "fav_number"
      t.decimal  "lucky_num",  precision: 2, scale: 2
      t.datetime "birthday"
      t.datetime "created_at"
      t.datetime "updated_at"
    end
  </code></pre>
  
  <p>I'd like to remove the <code>name</code> default value of <code>null: false</code>. I've tried running a separate migration with <code>change_column_default</code>, but that had no impact on <code>schema.rb</code>. Any suggestions?</p>