fix(driver): Compatibility with the registry 0.8

The docker-registry-core >= 2.0.0 (docker-registry >= 0.8) breaks the
compatibility with the config usage (docker/docker-registry#444)

This commit fixes bacongobbler/docker-registry-driver-swift#12

Signed-off-by: Vincent Giersch <vincent.giersch@ovh.net>
Merge pull request #16 from ovh/fix-compat-core-2.0

fix(driver): Compatibility with the registry 0.8