update to new ruamel version that works on python 3.7

Author: joezuntz

setup.py

@@ -9,7 +9,7 @@
         author_email='[email protected]',
         license='MIT',
         keywords = ['cwl'],
-        install_requires=['six', 'ruamel.yaml==0.13.13'],
+        install_requires=['six', 'ruamel.yaml==0.15.87'],
         packages=["cwlgen"],
         classifiers=[
             'Development Status :: 4 - Beta',