don’t try to insert extensions into ActiveRecord::Attribute if it doe…

…sn’t exist

lib/schema_plus.rb

@@ -153,7 +153,7 @@ def self.insert #:nodoc:
     ::ActiveRecord::Schema.send(:include, SchemaPlus::ActiveRecord::Schema)
     ::ActiveRecord::SchemaDumper.send(:include, SchemaPlus::ActiveRecord::SchemaDumper)
     ::ActiveRecord.const_set(:DB_DEFAULT, SchemaPlus::ActiveRecord::DB_DEFAULT)
-    ::ActiveRecord::Attribute.send(:include, SchemaPlus::ActiveRecord::Attribute)
+    ::ActiveRecord::Attribute.send(:include, SchemaPlus::ActiveRecord::Attribute) if defined? ::ActiveRecord::Attribute
   end
 
 end