Update gcs-oauth2-boto-plugin dependency to version 1.8

Update gcs-oauth2-boto-plugin dependency to version 1.8 to fix issue https://github.com/GoogleCloudPlatform/docker-registry/issues/17
Yes, I've tested it both on CoreOS in GCE as well as locally under VirtualBox.

The config option just tells boto that it's ok to look for a metadata server if no other credentials can be found. If credentials have already been located, then it won't be used.