builds: automatic build to get latest CentOS 7

[diegodelemos]

We need, for security reasons, to be up to date with the base CentOS image, so this are the options ordered by preference:

• Explore the possibility of using Docker Hub repository links so we would link with centos:7 and a rebuild would be triggered when the image changes.
• Add a daily Travis build which will trigger a rebuild on Docker Cloud via webhooks (to create the webhook https://cloud.docker.com/swarm/inveniosoftware/repository/docker/inveniosoftware/[IMG-NAME]/builds/edit go to the bottom of the page).
• Add a daily Travis build which would build the image itself and push it to Docker Cloud.