[rb] Update CHANGES for 3.13.0
+3.13.0 (2018-06-25)
+===================
+
+Ruby:
+ * Address warnings for redefined methods and uninitialized ivars
+
+Chrome:
+ * Chrome options capabilities updated to use goog:chromeOptions
+ * Added ability to direct headless chrome to save files (thanks @pelly)
+ * Change #download_files_to to #download_path
+
3.12.0 (2018-05-08)
===================
