tell verify_dependencies to be quieter

Running anything `kitchen`-related with kitchen-docker now produces lengthy stderr output, which makes it harder to see things.

Add redirection for STDERR when running the docker binary, as now running the `docker` command outputs to STDERR (version 1.2.0).
This is addressed in a [not-yet-released commit](https://github.com/docker/docker/commit/61b129d81802e3c988cc0e67e488b24968dd748a) where the `help` command is redirected.