Fix Rakefile generation to avoid circular dependency of tasks

Default generated Rakefile by 'serverspec-init' command causes an error with this procedure.
```bash
$ bundle exec serverspec-init
 ....
 + Rakefile
 ...
$ bundle exec rake
rake aborted!
Circular dependency detected: TOP => default => spec => spec:all => spec:default => spec:all

Tasks: TOP => default => spec => spec:all => spec:default
(See full trace by running task with --trace)
```
Probably we should see sample_spec.rb tests run with failure results instead.
This patch is to avoid this "Circular dependency."

Thanks for the great work of serverspec!