Fixing PostgreSQL migrations for array support

Since the `column` method is not used by the abstract `SchemaStatements::AlterTable`, the following migration will not work properly in PostgreSQL:

    class AddRolesToUsers < ActiveRecord::Migration
      def change
        change_table :users do |t|
          t.column :roles, :string, array: true
        end
      end
    end

This change fixes this issue.