Upgrade docker-client dependency
Fix error message in CLI

Fixes #264
Merge pull request #265 from spotify/dxia/fix-cli-err-msg

Fix error message in CLI
Fix bug where JobStatusCommand can throw NPE

This could happen when getting host status. We first get a list of all
jobs, then the status for each job. If a job was deleted between those
two steps, we would throw a NPE when trying to process its status.

-RN
Fixed bug where 'helios status --host' would sometimes throw NPE