Update Travis to use pretty Docker-based workers

These start up much faster and the only caveat is that we can't use "sudo" (which we don't currently use anyhow).

Also, I've updated the Go version here to match what's in the Dockerfile.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6884 from tianon/use-docker-based-travis

Update Travis to use pretty Docker-based workers