should be able to run ActiveModel validations_test [#4933 state:resolved]

Signed-off-by: José Valim <jose.valim@gmail.com>

activemodel/test/cases/helper.rb

@@ -6,6 +6,7 @@ $:.unshift(lib) unless $:.include?('lib') || $:.include?(lib)
 require 'config'
 require 'active_model'
 require 'active_support/core_ext/string/access'
+require 'active_support/core_ext/hash/except'
 
 # Show backtraces for deprecated behavior for quicker cleanup.
 ActiveSupport::Deprecation.debug = true