schema/manifest_backwards_compatibility_test: Fix gzip compat type
schema/manifest_backwards_compatibility_test: Fix gzip compat type

Catch up with aad7f24 (media-types: Define layer media types with and
without '+gzip', 2016-09-20, #388).

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

opencontainers/image-spec,vbatts/oci-image-spec,jonboulle/image-spec,mattspencer-arm/image-spec,jonboulle/image-spec,opencontainers/image-spec,mattspencer-arm/image-spec,vbatts/oci-image-spec