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 
line 30 of this file, sorry gh won't let me leave a comment on that line