Require hash/keys inside active_model/callbacks

This file uses assert_valid_keys but it was not being required. You can
reproduce this error with a script that uses this feature by using those
requires:

    require 'active_model'
    require 'active_model/callbacks'

activemodel/lib/active_model/callbacks.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "active_support/core_ext/array/extract_options"
+require "active_support/core_ext/hash/keys"
 
 module ActiveModel
   # == Active \Model \Callbacks