Move cwltool so it isn't a core dependency

Author: Peter Amstutz

setup.py

@@ -26,15 +26,15 @@
           'future',
           'connexion==1.4.2',
           'ruamel.yaml >= 0.12.4, <= 0.15.77',
-          'cwlref-runner==1.0',
-          'schema-salad >= 3.0, < 3.1',
+          'schema-salad',
           'subprocess32==3.5.2'
                         ],
       entry_points={
           'console_scripts': ["wes-server=wes_service.wes_service_main:main",
                               "wes-client=wes_client.wes_client_main:main"]
                     },
       extras_require={
+          "cwltool": ['cwlref-runner'],
           "arvados": ["arvados-cwl-runner"
                       ],
           "toil": ["toil[all]==3.18.0"