Fixed support of Rails 3.0.x rake tasks definition

In order current code is not support Rails 3.0.x.:

To define tasks sprocket-rails use `app`: https://github.com/jetthoughts/sprockets-rails/blob/master/lib/sprockets/railtie.rb#L59

But for rails `3.0.stable`: https://github.com/rails/rails/blob/3-0-stable/railties/lib/rails/railtie.rb#L185

There is no any `app` arguments for rake tasks blocks.