Bump minimum version requirement for ruamel.yaml
ruamel.yaml <= 0.16.6 suffers from a bug where ruamel's yaml dumper crashes when used on a sequence that has comments before it. In BuildStream, this manifests in form of issues like #1265 (closed).
See upstream issue at https://sourceforge.net/p/ruamel-yaml/tickets/335.
Also, add a regression test for it.
Fixes #1265 (closed).
Edited by Chandan Singh