Make `apt-key` work from behind firewall

Hi! I had trouble getting the apt recipe to work from our company network, and I encountered this blog post: http://lodge.glasgownet.com/2012/07/11/apt-key-from-behind-a-firewall/. Its author describes that including a schema and port for the `--keyserver` parameter fixes things. I made a change in repository.rb accordingly and indeed, Things were Fixed (tm).

My understanding is that this change can never hurt, but admittedly I am not completely familiar with the impact of this change. I see that two lines above, when proxy settings are given, you do include a schema and port.

If you agree that this is a non-breaking change, will you accept my pull request? Thanks!