replace rake/rdoctask with rdoc/task

I encountered an error when I ran `rake test` (Ruby version: 1.9.3-p448)

![screen shot 2013-08-26 at 7 56 52 pm](https://f.cloud.github.com/assets/5047/1025550/af79437c-0e47-11e3-97c5-2129bafab3df.png)