replace deprecated __xonsh_history__ with __xonsh__.history

xonsh:29: DeprecationWarning: __xonsh_history__ has been deprecated, please use __xonsh__.history instead.

rc.xsh

@@ -24,7 +24,7 @@ xontribs_load(['vox', 'z'])
 
 def _last_exit_status():
     try:
-        exit_status = __xonsh_history__.rtns[-1]
+        exit_status = __xonsh__.history.rtns[-1]
         return exit_status if exit_status != 0 else None
     except IndexError:
         return None