Add simple integration tests and integrate them into the CI.
More complex tests will be added later.
Fixes: #15
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>
Enable a simple CI via Travis which currently install skopeo and
shellcheck and runs `make validate`.
Fixes: #13
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>