version: Change default version name to `dev`
version: Change default version name to `dev`

This change makes the image associated with the master branch have the
tag `dev`. This is better because it matches the tag we use to track the
HEAD of the master branch. Furthermore, this will cleanly fix a build bug
introduced by `travis: Push Docker images for tags rather than branches`
because we won't require special logic for the `master` branch.

quilt/quilt,kklin/kelda,ejj/quilt-dev,NetSys/quilt,kklin/quilt,quilt/quilt,kelda/kelda,kklin/quilt,kklin/quilt,kayousterhout/quilt,LuiseV/quilt,ejj/quilt-dev,kelda/kelda,ejj/quilt-dev,LuiseV/quilt,NetSys/quilt,ejj/quilt-dev,quilt/quilt,NetSys/quilt,LuiseV/quilt,kayousterhout/quilt,kklin/quilt,LuiseV/quilt,NetSys/quilt,kklin/kelda,kayousterhout/quilt,kklin/kelda,NetSys/quilt,quilt/quilt,kelda/kelda,kklin/kelda,LuiseV/quilt,kayousterhout/quilt,kelda/kelda