bump to v0.1.20
bump to v0.1.20

- support image-spec v1.0.0-rc5

Signed-off-by: Antonio Murdaca <f9809039ea4850e75dcb8d80b778844aa463358f@redhat.com>

mtrmac/skopeo,projectatomic/skopeo,jwhonce/skopeo,jwhonce/skopeo,mtrmac/skopeo,projectatomic/skopeo,mtrmac/skopeo