Fix Travis handling of a fully-numeric commit hash being YAML-converted to a number instead of a string

Docker-DCO-1.0-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3491 from tianon/travis-numeric-commit

Fix Travis handling of a fully-numeric commit hash being YAML-converted ...
Only get rootfs when we need to calculate the image size

Docker-DCO-1.0-Signed-off-by: Michael Crosby <crosby.michael@gmail.com> (github: crosbymichael)