Build ruamel.yaml 0.15.26 for a-c-r

Build ruamel.yaml 0.15.26 for a-c-r

This is last version before a serious regression was introduced in
parsing flow mapping

https://bitbucket.org/ruamel/yaml/issues/227/regression-parsing-flow-mapping

This affects 0.15.27 to 0.15.51

0.15.52 haven't been tested because they have a different
compatibility problem:

https://github.com/common-workflow-language/schema_salad/pull/195

refs #14075

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <pamstutz@veritasgenetics.com>