Pin ruamel.yaml version to <= 0.15

Description

As per #571 (closed), BuildStream may stop working when we run a pip3 install --user -e . due to a new upstream version of ruamel. Pinning to <= 0.15 is ruamel's recommendation for production systems.

Proposed changes

Pin ruamel verison to <= 0.15 in setup.py

This merge request, when approved, will close issue/bug: #571 (closed)