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 
that's docs.docker.io. I can change that if @SvenDowideit @fredlf or @jamtur01 are fine with it, but this PR is about get.docker.io and test.docker.io.