Use keyserver on port 80 for better proxy/firewall interop.

Typical corporate web proxy filter out rare ports like
the default port for the key server. This change advises
add-apt-repository to use hkp://keyserver.ubuntu.com:80
Merge pull request #20 from bentolor/fix/keyserver-port-interoperability

Use keyserver on port 80 for better proxy/firewall interop.