rake/rdoctask is deprecated

We need to use rdoc/task instead:

````
$ rake
rake aborted!
ERROR: 'rake/rdoctask' is obsolete and no longer supported. Use 'rdoc/task' (available in RDoc 2.4.2+) instead.
/home/frank/Documents/rantly/Rakefile:42:in `<top (required)>'
(See full trace by running task with --trace)

````