Stop using dockerfile/* images

As per
http://blog.docker.com/2015/03/updates-available-to-popular-repos-update-your-images/
docker has stopped answering dockerfile/redis and dockerfile/nginx.  Fix all
users in our tree.  Sadly this means a lot of published examples are now broken.

examples/k8petstore/redis/Dockerfile

@@ -5,7 +5,7 @@
 #
 
 # Pull base image.
-FROM dockerfile/ubuntu
+FROM ubuntu
 
 # Install Redis.
 RUN \