Replace get.docker.io -> get.docker.com and test.docker.io -> test.docker.com

Signed-off-by: Tibor Vass <teabee89@gmail.com>

This is in the context of migrating to get.docker.com.

Ping @tianon @crosbymichael @vieux @SvenDowideit @jamtur01 @fredlf 
@crosbymichael @vieux what about get-staging.docker.io ? Is that something we also need to migrate? or should that be test.docker.com ?