Update version to 2.1.3-beta.0, enhance command_executor_url (#188)

Author: prklm10

.gitignore

@@ -5,3 +5,4 @@ build
 *.log
 **/__pycache__
 node_modules*
+.DS_Store

percy/driver_metadata.py

@@ -12,7 +12,13 @@ def session_id(self):
     def command_executor_url(self):
         url = Cache.get_cache(self.session_id, Cache.command_executor_url)
         if url is None:
-            url = self.driver.command_executor._url # pylint: disable=W0212
+            try:
+                url = self.driver.command_executor._url  # pylint: disable=W0212
+            except (AttributeError, Exception):
+                url = (
+                    self.driver.command_executor.client_config.remote_server_addr
+                )  # pylint: disable=W0212
+
             Cache.set_cache(self.session_id, Cache.command_executor_url, url)
             return url
         return url

percy/version.py

@@ -1 +1 @@
-__version__ = '2.1.2'
+__version__ = "2.1.3-beta.0"