force co = 3.0.2

I have ```co-3.0.2``` both as my dependency and as a koa dependency. 
While trying to upgrade my project dependencies (to see if there was any improvement) I upgraded to co-3.0.4 which totally broke anything I then had to go back to 3.0.2 .

I also upgraded koa to 0.5.1, but doing so pulled ```co-0.3.4``` as a koa dependecy (due to the ```~``` )breaking again anything.

It seems to me that there is a “compatibility” issue between ```co-0.3.2``` and ```co-0.3.4``` that prevents some koa installations (e.g. mine) to correctly work. It would be great if you can accept my pull request so i don't have to do this by hand or i don't have to put this fork as a dependency :)