lib/rockin/recipes/nginx.rb: Assume yes during add-apt-repository

Without this you run into this particular error
   * 2013-01-14 12:45:39 executing `nginx:install'
   * executing "sudo -p 'sudo password: ' add-apt-repository ppa:nginx/stable"
     servers: ["ajscg.com"]
     [ajscg.com] executing command
  ** [out :: ajscg.com] You are about to add the following PPA to your system:
  ** [out :: ajscg.com] Stable version of nginx.
  ** [out :: ajscg.com] More info: https://launchpad.net/~nginx/+archive/stable
  ** [out :: ajscg.com] Press [ENTER] to continue or ctrl-c to cancel adding it

Signed-off-by: Adam Stokes <hackr@cypherbook.com>