codeclimate/codeclimate-rubocop

This cop checks for add_column call with NOT NULL constraint
in migration file.

### Example:
    # bad
    add_column :users, :name, :string, null: false
    add_reference :products, :category, null: false

    # good
    add_column :users, :name, :string, null: true
    add_column :users, :name, :string, null: false, default: ''
    add_reference :products, :category
    add_reference :products, :category, null: false, default: 1