Fix docker_inspect_id for docker 1.0+

This fixes docker_inspect_id for docker 1.0+. The JSON format for
`docker inspect` changed, and this supports the new format in a
backwards-compatible way.

This should fix issues with notifications from a `docker_image` with
`action :pull` by properly detecting changes in the id.
Merge pull request #166 from jcrobak/fix-id-detection

Fix docker_inspect_id for docker 1.0+