[:alpha available] Feature/force update

[rusenask]

Changed:

• Migrated dependency management from Glide to Dep

Fixed:

• fix for Zalando Registry gives seemingly erroneous 404's #153, for some reason in the docker client we were using HEAD instead of GET.
• fix for Unversioned deployment with multiple pods does not update all pods #110, instead of changing image tag to 0.0.0 (which worked for some time) now force policy (for same image tags such as latest) will terminate all pods for that deployment. Make sure you set image pull policy to always. Optional variable for shutdown grace period is supplied through annotation keel.sh/gracePeriod=10 (defaults to 5 seconds).

Updated:

• Updated k8s client-go, api and apimachinery to release-0.6 branch. This required some changes in k8s provider library but in the end should be a good call as k8s client update was overdue already.
• Removed Ping() function from the registry client as that endpoint is not reachable and was more or less useful just for debugging purposes.