Merge pull request #917 from common-workflow-language/ruamel_pin

fix ruamel version

Author: mr-c

setup.py

@@ -53,7 +53,7 @@
           'setuptools',
           'requests >= 2.6.1',  # >= 2.6.1 to workaround
           # https://github.com/ionrock/cachecontrol/issues/137
-          'ruamel.yaml >= 0.12.4, < 0.16',
+          'ruamel.yaml >= 0.12.4, <= 0.15.51',
           'rdflib >= 4.2.2, < 4.3.0',
           'shellescape >= 3.4.1, < 3.5',
           'schema-salad >= 2.7.20180905124720, < 3',