Bump containers/storage and containers/image, and pin them to their
current versions. This requires that we update image-spec to rc6 and
add github.com/ostreedev/ostree-go, which adds build-time requirements
on glib2-devel and ostree-devel.
Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>
Closes: #174
Approved by: rhatdan