ref(client): remove PyYAML dependency and save to ~/.deis/client.json
fix(client): convert client.yaml to client.json once if needed

Our use of YAML was very restricted, so a set of python string
replace calls converts it to valid JSON without requiring PyYAML.
If that succeeds, delete the old ~/.deis/client.yaml file, otherwise
ignore errors and just use ~/.deis/client.json from now on.
Merge pull request #1767 from mboersma/1749-no-pyyaml

ref(client): remove PyYAML dependency and save to ~/.deis/client.json