Rephrase to remove obsolete Docker Store, Docker Cloud. (#184)
Rephrase to remove obsolete Docker Store, Docker Cloud. (#184)

Also, replace [Docker Cloud](https://cloud.docker.com) to [Docker Hub](https://hub.docker.com).

To fix #183.
play-with-docker/play-with-docker.github.io,play-with-docker/play-with-docker.github.io