fix ruamel.yaml on 0.14.X for now

[AvdN]

Thank you for using ruamel.yaml.

There will be API changes in the 0.15+ versions, that might lead to warnings that
your users could see.
According to PyPI there is no fix on a version number, and if I install your package in a virtualenv with pip I get ruamel.yaml 0.15.0. Maybe you (also) have to update setup.py file ( install_requires, automatically read in from requirements.txt?)