Push a new docker image with tag "master" for every new build on the
master branch. This gives users wishing to run with the latest
development changes an easy way to stay up to date.

Note: when a tag is created, Travis sets the $TRAVIS_BRANCH envvar to
the tag, rather than the branch, hence why we only use this variable
now.

This will correspond to to tagged versions for Marge, so users will be
able to use the latest versioned update automatically.

Explain how we tag our images and the recommended approach.

By default, we want users to get a stable version. If they want the
latest updates, they can use the master tag instead.