undefined method `add_to_rails_paths' for #<Module:0x3f72698

Hi,
I have added "config.gem "pdf-writer", :version=>'1.1.8'" in my
application's environment.rb so that I can use "rake gems:install" for
installing all gems.

My problem is whenever I am running the command "rake gems:install" in
application's dir it is giving me following error:-
"undefined method `add_to_rails_paths' for #<Module:0x3f72698>"

Anyone have any idea how to solve this?

Thanks,
Mike