Travis to build docker images and push to docker hub

[karthikb351]

Docker hub is free for public repositories and supports GitHub auto-builds, but the time taken to build new images is quite slow because of the queue (around 30 minutes).

A faster way would be to use travis to build an imagine on every commit and push that to docker hub.

The other alternative to host our own prive docker registry that does the image builds for us, but this is quite expensive and will be an additional thing to maintain.