Fix `tasks` command bug with rake >= 0.9.0

`tasks` command fails with rake >= 0.9.0.

```
rails-sh(foo)> tasks
Unknown show task mode: ''
/Users/labocho/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-10.1.0/lib/rake/application.rb:283:in `display_tasks_and_comments'
/Users/labocho/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rails-sh-1.5.2/lib/rails/sh/commands.rb:43:in `block in <top (required)>'
/Users/labocho/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rails-sh-1.5.2/lib/rails/sh.rb:51:in `call'
/Users/labocho/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rails-sh-1.5.2/lib/rails/sh.rb:51:in `execute'
/Users/labocho/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rails-sh-1.5.2/lib/rails/sh.rb:28:in `start'
/Users/labocho/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rails-sh-1.5.2/bin/rails-sh:28:in `<top (required)>'
/Users/labocho/.rbenv/versions/2.0.0-p0/bin/rails-sh:19:in `load'
/Users/labocho/.rbenv/versions/2.0.0-p0/bin/rails-sh:19:in `<main>'
```

Newer rake requires some configurations for showing tasks.