Updating rakefile

I have modified those, because some class is no longer supported.


```
$ bundle exec rake -T
aborted!
ERROR: 'rake/rdoctask' is obsolete and no longer supported. Use 'rdoc/task' (available in RDoc 2.4.2+) instead.
```

and 

```
$ bundle exec rake -T

rake aborted!
ERROR: 'rake/gempackagetask' is obsolete and no longer supported. Use 'rubygems/packagetask' instead.
```