Update taskclusterrc to use the latest ubuntu-ci image
Update taskclusterrc to use the latest ubuntu-ci image

mythmon/taskcluster-github,rail/taskcluster-github,rail/taskcluster-github,petemoore/taskcluster-github,mrrrgn/taskcluster-github,petemoore/taskcluster-github,taskcluster/taskcluster-github,mythmon/taskcluster-github,mrrrgn/taskcluster-github