Update travis job to use git tags for docker container tags

[disrani-px]

Is this a BUG REPORT or FEATURE REQUEST?:
Release improvement

What happened:
Need to manually tag and push images for releases right now

What you expected to happen:
We should automatically use github tags for the docker container tags. This will make it easier to publish releases. All we'll need to do is create a git tag and it'll use it for the container tag and push to docker hub.