Fix build authentication header for API version >= 1.19

[csrwng]

The format for the map used by Docker to read auth configs on the BuildImage API call has changed twice since Docker 1.6.x (API version 1.18):

In Docker 1.6.2 (API version 1.18), it uses this format:

{ "configs": { auth config map } }

Defined here: https://github.com/docker/docker/blob/v1.6.2/registry/auth.go#L37-L40
Decoded here: https://github.com/docker/docker/blob/v1.6.2/api/server/server.go#L1043

In Docker 1.7.0 (API version 1.19), it uses this format:

{ "auths": { auth config map } }

Defined here: https://github.com/docker/docker/blob/v1.7.0/cliconfig/config.go#L41-L45
Decoded here: https://github.com/docker/docker/blob/v1.7.0/api/server/server.go#L1218

In Docker 1.7.1 and later (still API version 1.19), it uses this format:

{ auth config map }

Decoded here: https://github.com/docker/docker/blob/v1.7.1/api/server/server.go#L1227

This PR allows go-dockerclient to support the most recent format based on detecting API version 1.19 and above. It ignores the format in Docker 1.7.0 since it was there only for a single version and it was quickly replaced in 1.7.1 (and there wasn't a change in API version)

[csrwng]

@bparees @ncdc fyi

[fsouza]

Nice catch, thanks!