Travis: add vendor checks
Travis: add vendor checks

Add checks to Tarvis to make sure that the vendor.conf is in sync with
the code and the dependencies in ./vendor.  Do this by first running
`make vendor` followed by running `./hack/tree_status.sh` to check if
any file in the tree has been changed.

Signed-off-by: Valentin Rothberg <c8d7c67387706f1534261b1aba83f5c355c00ff5@redhat.com>

mtrmac/skopeo,mtrmac/skopeo,mtrmac/skopeo