Correct AddDefaultOtpRequiredForLoginValue migration

Also MySQL complains when you change a field to be not null if any
existing records already have a null value, so this updates those rows.

db/migrate/20150620233230_add_default_otp_required_for_login_value.rb

 class AddDefaultOtpRequiredForLoginValue < ActiveRecord::Migration
   def up
+    execute %q{UPDATE users SET otp_required_for_login = FALSE WHERE otp_required_for_login IS NULL}
+
     change_column :users, :otp_required_for_login, :boolean, default: false, null: false
   end
 
   def down
-    change_column :users, :otp_required_for_login, :boolean, default: nil
+    change_column :users, :otp_required_for_login, :boolean, null: true
   end
 end