Fix ActiveModel::Errors deprecation messages failing when used on its own

  Deprecation messages in ActiveModel::Errors are using String#squish
from ActiveSupport but were not explicitly requiring it, causing failures
when used outside rails.

Author: radanskoric

activemodel/lib/active_model/errors.rb

@@ -3,6 +3,7 @@
 require 'active_support/core_ext/array/conversions'
 require 'active_support/core_ext/string/inflections'
 require 'active_support/core_ext/object/deep_dup'
+require 'active_support/core_ext/string/filters'
 
 module ActiveModel
   # == Active \Model \Errors