Make Travis push the images to the Docker hub if the tests succeed

[LuisAlejandro]

If #109 gets implemented, automated builds on the docker hub will need to be deprecated because a clone of a private repository will fail there. One solution to this problem could be to use Travis builds to push the images to hub.docker.com if they pass all tests (after #111 is done). This can be easily done by storing push credentials in the Travis encrypted tokens.

[LuisAlejandro]

I've been implementing this feature on my personal images. Feel free to copy the implementation.

The relevant parts are:

• Login data for hub.docker.com (encrypted on .travis.yml).
• A matrix containing all the images to build.
• A script to build the images in the script section of .travis.yml.
• An after_success script in .travis.yml that pushes to hub.docker.com.

This last step should probably check for ${TRAVIS_BRANCH} == master to avoid pushing unstable images from pull requests or other branches.

[moylop260]

Thanks!
We are using a new propagate option from hub.docker.com
If we have a image changed from repository then build image from docker hub and propagate the build of children images