Rename Docker images to remove the '.'

On Docker 0.10.0, the CentOS images won't build with a '.' in the image name:

$ docker build -t titotest-centos-6.4 .
2014/05/08 11:02:05 Invalid repository name (ex: "registry.domain.tld/myrepos")
$ docker build -t titotest-centos-6-4 .
Uploading context 3.584 kB
Uploading context
Step 0 : FROM tianon/centos:6.4
 ---> f6a805acbe08