Fix regression with ping endpoint (#484)

Author: savingoyal

metaflow/plugins/metadata/service.py

@@ -260,7 +260,7 @@ def _version(cls, monitor):
         if cls.INFO is None:
             raise MetaflowException('Missing Metaflow Service URL. '
                 'Specify with METAFLOW_SERVICE_URL environment variable')
-        path = 'ping/'
+        path = 'ping'
         url = os.path.join(cls.INFO, path)
         for i in range(METADATA_SERVICE_NUM_RETRIES):
             try: