Fix ActiveRecord version check in Rails 4.0

Rails 4.0 does not have the `ActiveRecord.gem_version` method, so this
check will fail there. However, we know that if `gem_version` is not
available, we are not dealing with Rails >= 5.0, so this will work in
both cases.

See paper-trail-gem#956 for an equivalent case elsewhere in the codebase.

lib/paper_trail/version_concern.rb

@@ -12,7 +12,8 @@ module VersionConcern
     extend ::ActiveSupport::Concern
 
     included do
-      if ActiveRecord.gem_version >= Gem::Version.new("5.0")
+      if ::ActiveRecord.respond_to?(:gem_version) &&
+          ::ActiveRecord.gem_version >= Gem::Version.new("5.0")
         belongs_to :item, polymorphic: true, optional: true
       else
         belongs_to :item, polymorphic: true