Push 'latest' tag to Docker Hub

[arm4b]

Somewhat related to #31

Start pushing latest Docker tags to Docker hub so users could use that instead of relying on pinned st2 version.

Gotchas:

• Do version sort, upload latest tag based on most recent real version
• Make sure to exclude dev versions, only st2 stable versions fit to be latest.
• Some scripting examples from the st2-docker project should help: https://github.com/StackStorm/st2-docker/blob/927de5adfcd5c5e95bc811a0839ed998f541b436/bin/deploy.sh#L40-L43

[arm4b]

Implemented by #41